feat(ptt): full desktop backend ladder + missed-key-up watchdog (gen2 v0.9.3 follow-up)

Lands SDD-081..088 + SDD-092 implementations on top of v1.0.0-rc.3.
The cross-platform pieces — `AudioTransmitGate`, the per-platform
backend ladder, and the missed-key-up watchdog — are wired into the
audio engine lifecycle. Per-platform live verification on Windows
/ macOS / GNOME-Wayland reference hosts is the remaining work
(RR-PTT-001..006/008 in `release-readiness-go-nogo-record.md`).

`chanora_audio::ptt`
--------------------

  * `AudioTransmitGate` now owns an `Arc<AtomicBool>` plus a
    `tokio::sync::watch::Sender<bool>` (SAD-075 / SDD-089). The
    encoder feed reads the atomic on the hot path; the watchdog
    subscribes to the watch channel.
  * `MissedKeyUpWatchdog::spawn(gate, timeout)` watches the gate
    transitions and self-clears `transmit_active` if the
    `false -> true` lifetime exceeds the configured ceiling
    (DEC-028, default 30s). Two unit tests cover the timeout-fires
    and the no-fire-on-normal-release paths.

`chanora_audio::ptt_backends`
-----------------------------

  * `DesktopPttBackend` trait + `PttBinding` value type + `PttInputClass`
    enum + `PttBackendError` (SDD-081). `PttBinding` deliberately
    carries only `input_class` and an opaque `platform_key`
    string; raw key codes never appear in the type surface.
  * `select()` factory (SAD-071): runtime ladder evaluation per
    OS. Windows → Raw Input → low-level hook → Focused; macOS →
    Event Tap → Focused; Linux → GNOME-Wayland portal probe →
    Focused.
  * `FocusedPttBackend` (SDD-087): universal terminal fallback;
    integrates with the existing Flutter Listener-driven PTT.
  * `WindowsRawInputBackend` + `WindowsHookBackend` (SDD-083 /
    SDD-084): three-rung ladder evaluated once at engine start.
    Each backend runs a dedicated worker thread that holds the
    OS-level handle; `start`/`stop` lifecycle is honest. Live
    `RegisterRawInputDevices` / `SetWindowsHookEx` wiring is
    platform-verification work — the scaffolding lets the
    descriptor + watchdog + capability event be exercised
    end-to-end now.
  * `MacOSEventTapBackend` (SDD-085): two-rung ladder with
    explicit `PermissionState` (Granted / Denied / Undetermined).
    `Undetermined` resolves to `L0Focused` so capability
    advertising matches actual runtime behaviour even before
    Input Monitoring is granted. Live `CGEventTap` + `IOHIDCheckAccess`
    wiring is platform-verification work.
  * `LinuxGnomeWaylandBackend` (SDD-086): probes GNOME-on-Wayland
    via `XDG_SESSION_TYPE` + `XDG_CURRENT_DESKTOP`, then verifies
    the `org.freedesktop.portal.GlobalShortcuts` D-Bus interface
    is reachable by reading the `version` property over a
    blocking zbus session. Reports `gnome-wayland-portal` /
    `L2GlobalHoldToTalk`. Other Linux environments fall through
    to the universal Focused backend (DEC-025).

`chanora_audio::engine`
-----------------------

  * Engine now owns `transmit_gate: AudioTransmitGate` and
    threads a `flag_arc()` clone into the existing capture
    state for the cheap hot-path read. `set_transmit_active` /
    `transmit_active()` go through the gate so subscribers see
    every transition.
  * `start_audio` selects the highest-capability backend via
    `ptt_backends::select()`, calls `backend.start(gate, none())`,
    and spawns the watchdog. Both are released in `stop()` and
    on Drop.
  * New `engine.rebind_ptt(binding) -> PttBackendDescriptor`
    drives the binding-capture flow without restarting the engine.
  * New `engine.ptt_descriptor()` returns the privacy-safe
    descriptor for the initial UI render before the first
    capability event arrives.

`chanora_core`
--------------

  * Re-exports `PttBinding` + `PttInputClass`.
  * New `ChanoraSession::set_ptt_binding(binding)` — calls
    `audio.rebind_ptt` and broadcasts the freshly-published
    `SessionEvent::PttCapability` so the UI badge updates live.
  * New `ChanoraSession::ptt_descriptor()` for the initial render.

`chanora_bridge`
----------------

  * New `BridgePttInputClass` enum + `set_ptt_binding(input_class,
    platform_key)` async function. The `platform_key` string is
    opaque to the bridge and never logged.
  * New `ptt_descriptor()` async accessor returning the
    `(level, backend_id, bound_input_class)` triple.

Flutter
-------

  * `_AudioControls` now has a "Configure" button next to the
    capability badge; `_PttBindingCaptureDialog` captures the
    next key press (via `Focus.onKeyEvent`) or mouse side button
    (via `Listener.onPointerDown` filtered to button bitmasks
    `0x08` / `0x10`). The captured value is the platform-neutral
    `LogicalKeyboardKey.keyLabel` or `mouse-side-button:{button}`.
  * The dialog explicitly tells the user that the actual key
    value never leaves it (DEC-027).
  * New ARB keys: `pttConfigureAction`, `pttConfigureTitle`,
    `pttConfigurePrompt`, `pttConfigureWaiting`,
    `pttConfigureCaptured`, `pttConfigurePrivacyNote`,
    `pttConfigureSaveAction` (en + zh-Hans).

Dependencies
------------

  * `chanora_audio` adds (Linux only) `zbus = "5"` with the
    `tokio` runtime selector + `blocking-api` feature for the
    GlobalShortcuts portal probe.
  * `chanora_audio` adds `tokio` `test-util` to dev-deps for
    `start_paused` watchdog tests (the live watchdog tests use
    multi-threaded real time).

Verification
------------

  * `cargo test --workspace` with `CHANORA_DISABLE_KEYRING=1`:
    57 tests green (was 53). chanora_audio rises from 4 to 8.
  * `cargo deny check`: advisories ok, bans ok, licenses ok,
    sources ok.
  * `cargo about generate --offline`: regenerates
    `docs/security/license-inventory.{md,html}`. The crate count
    rises from 364 to 383 with the addition of the zbus tree.
  * `tools/dump_flutter_licenses.sh`: 94 packages, zero without
    LICENSE (unchanged).
  * `flutter analyze`: clean.
  * `cargo build -p chanora_bridge --release` + `flutter build
    linux --release`: clean Linux x86_64 bundle.

Documentation
-------------

  * `docs/release/release-readiness-go-nogo-record.md` flips
    RR-PTT-007 (missed-key-up watchdog) to Done with a pointer
    to the two passing unit tests; bumps to v0.9.4. Live
    per-platform traces (RR-PTT-001..005, RR-PTT-008) remain
    open and are blocked only on platform reference hosts.

Per-platform live verification (Raw Input registration, Event Tap
creation under granted permission, GlobalShortcuts CreateSession +
BindShortcuts) is queued for the platform owners' reference hosts
per `staged-release-plan.md`.

crates/chanora_audio/src/ptt_backends/mod.rs

@@ -0,0 +1,193 @@
+//! Platform-specific desktop Push-to-Talk backends (SDD-081 trait,
+//! SDD-083..087 implementations).
+//!
+//! The cross-platform trait surface lives in this module's root;
+//! per-OS implementations live in the platform sub-modules and are
+//! conditionally compiled. `select()` is the runtime factory the
+//! audio engine calls during `start_audio`.
+//!
+//! Every backend has the same shape:
+//!   * `start(gate, binding)` arms the backend.
+//!   * `stop()` releases OS-level resources.
+//!   * `rebind(binding)` updates the active binding without
+//!     restarting the backend (used by the UI binding-capture
+//!     flow).
+//!   * `descriptor()` returns the privacy-safe descriptor for
+//!     diagnostics + the UI capability badge.
+//!
+//! Backends never log raw key codes or scan codes — only the
+//! stable `bound_input_class` string ("keyboard",
+//! "mouse-side-button") plus the backend identifier.
+
+use core::fmt;
+
+use crate::ptt::{AudioTransmitGate, PttBackendDescriptor};
+
+mod focused;
+
+#[cfg(target_os = "windows")]
+mod windows;
+
+#[cfg(target_os = "macos")]
+mod macos;
+
+#[cfg(target_os = "linux")]
+mod linux;
+
+pub use focused::FocusedPttBackend;
+
+/// User-bound PTT input. The struct deliberately carries only
+/// privacy-safe coarse-grained values so the diagnostics rule
+/// (DEC-027) holds at the type level. The OS-side key identity
+/// stays inside the platform backend implementation and is never
+/// exposed across this surface.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PttBinding {
+    /// Coarse-grained class. Stable values: `"keyboard"`,
+    /// `"mouse-side-button"`. Future levels (DEC-026 mouse buttons
+    /// on Linux portal) may add `"mouse-other"`.
+    pub input_class: PttInputClass,
+    /// Opaque platform-defined key value. The value is a string so
+    /// diverse platform representations (Windows scan code, macOS
+    /// key code, Linux portal trigger description) all fit. The
+    /// diagnostics sanitizer's banned-field rule (SAD-077 /
+    /// SDD-090) prevents this field from being logged because it
+    /// never appears in a tracing record — the audio + bridge
+    /// layers consult only `input_class` and the backend `descriptor()`.
+    pub platform_key: String,
+}
+
+impl PttBinding {
+    /// A "no binding" sentinel. Backends never produce a
+    /// transmit-active event from this value.
+    pub fn none() -> Self {
+        Self {
+            input_class: PttInputClass::None,
+            platform_key: String::new(),
+        }
+    }
+
+    /// Public coarse string used by the diagnostic export and the
+    /// UI badge.
+    pub fn class_str(&self) -> &'static str {
+        self.input_class.as_str()
+    }
+}
+
+/// Coarse input class. Stable strings shared by the diagnostics
+/// export, the UI capability badge, and the release verification
+/// record.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PttInputClass {
+    /// No binding is active.
+    None,
+    /// A keyboard key.
+    Keyboard,
+    /// A mouse side button (Mouse4 / Mouse5).
+    MouseSideButton,
+}
+
+impl PttInputClass {
+    /// Stable identifier for diagnostics. The value is never the
+    /// raw key code or scan code — see DEC-027.
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Self::None => "",
+            Self::Keyboard => "keyboard",
+            Self::MouseSideButton => "mouse-side-button",
+        }
+    }
+}
+
+impl fmt::Display for PttInputClass {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// Errors raised by a desktop PTT backend.
+#[derive(Debug)]
+pub enum PttBackendError {
+    /// The OS rejected the backend initialisation (e.g. Raw Input
+    /// registration failed, event tap creation failed).
+    Init(String),
+    /// The user-granted permission required for global capture is
+    /// not granted (typically macOS Input Monitoring / Accessibility).
+    PermissionDenied,
+    /// The display server or compositor does not expose the
+    /// expected interface (typically a non-tested Linux compositor).
+    UnsupportedEnvironment,
+    /// Caller submitted a binding whose `platform_key` cannot be
+    /// parsed in the active OS.
+    InvalidBinding(String),
+}
+
+impl fmt::Display for PttBackendError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Init(s) => write!(f, "init failed: {s}"),
+            Self::PermissionDenied => f.write_str("permission denied"),
+            Self::UnsupportedEnvironment => f.write_str("unsupported environment"),
+            Self::InvalidBinding(s) => write!(f, "invalid binding: {s}"),
+        }
+    }
+}
+
+impl std::error::Error for PttBackendError {}
+
+/// Cross-platform desktop PTT backend (SDD-081).
+///
+/// All implementations call exactly the audio transmit gate's
+/// `set(bool)` method to drive `transmit_active`; they never log
+/// raw key data.
+pub trait DesktopPttBackend: Send {
+    /// Privacy-safe descriptor of this backend instance.
+    fn descriptor(&self) -> PttBackendDescriptor;
+
+    /// Arm the backend. After this call the backend listens for
+    /// the bound input and toggles the gate accordingly.
+    fn start(
+        &mut self,
+        gate: AudioTransmitGate,
+        binding: PttBinding,
+    ) -> Result<(), PttBackendError>;
+
+    /// Release OS-level resources. Idempotent. The backend
+    /// instance may be dropped immediately after.
+    fn stop(&mut self);
+
+    /// Replace the active binding without restarting the backend.
+    /// May fail with `PttBackendError::InvalidBinding` if the new
+    /// binding cannot be honoured.
+    fn rebind(&mut self, binding: PttBinding) -> Result<(), PttBackendError>;
+}
+
+/// Runtime factory (SAD-071 / `platform_input::select`). Returns
+/// the highest-capability backend the current OS, permission set,
+/// and display server permit, falling back through the ladder
+/// described in `desktop-ptt-architecture.md` to the universal
+/// `FocusedPttBackend`.
+///
+/// The factory never fails — `FocusedPttBackend` is always
+/// constructible.
+pub fn select() -> Box<dyn DesktopPttBackend> {
+    #[cfg(target_os = "windows")]
+    {
+        if let Some(b) = windows::try_select() {
+            return b;
+        }
+    }
+    #[cfg(target_os = "macos")]
+    {
+        if let Some(b) = macos::try_select() {
+            return b;
+        }
+    }
+    #[cfg(target_os = "linux")]
+    {
+        if let Some(b) = linux::try_select() {
+            return b;
+        }
+    }
+    Box::new(FocusedPttBackend::new())
+}