feat(ptt): code-side initial split — transmit_active / capability badge / sanitizer

Implements the gen2 v0.9.3 doc baseline's first slice of code work:

  * SRS-201: split the audio engine's `ptt` AtomicBool into the
    authoritative `transmit_active` flag. The legacy `set_ptt` /
    `ptt` accessors are retained as `#[doc(hidden)]` thin wrappers
    so the existing bridge command and the existing Flutter
    hold-to-talk UI keep compiling.
  * SAD-075 / SDD-089 acknowledged at the type level: only
    `AudioEngine::set_transmit_active` (or its legacy alias)
    mutates the flag; the encoder feed reads it once per outbound
    frame and never writes.
  * SDD-082: new `chanora_audio::ptt` module ships the
    `PttCapabilityLevel` enum (`L0Focused`, `L1GlobalShortcut`,
    `L2GlobalHoldToTalk`, `L3GlobalWithMouseButtons`,
    `L4DeviceAware` reserved) with a stable `as_str` mapping and
    an `is_global` classifier.
  * SDD-087: `PttBackendDescriptor::focused()` constant value for
    the universal Focused-PTT fallback. The struct shape carries
    only privacy-safe fields (`level`, `backend_id`,
    `bound_input_class`) — a key code cannot fit through this
    surface by construction (DEC-027).
  * SAD-077 / SDD-090: `RedactingLogLayer` now hosts the
    `PttBanCheckVisitor` and the `PTT_BANNED_FIELDS` constant
    (`key_code`, `scan_code`, `virtual_key`, `vk`, `keysym`,
    `keysym_string`, `key_sequence`, `key_press_history`,
    `key_timing`). Any record whose field set names a banned key
    is dropped before reaching the in-memory log sink or the
    user-initiated diagnostic export. The check is structural and
    runs ahead of formatting / redaction.
  * `SessionEvent::PttCapability` carries the diagnostics-safe
    descriptor through the broadcast event stream;
    `chanora_core::ChanoraSession::start_audio` publishes the
    Focused-PTT descriptor when the audio engine starts (SRS-196
    / SDD-091).
  * `BridgeEvent::PttCapability` mirrors the event across the
    FFI boundary. flutter_rust_bridge codegen regenerated.
  * Flutter `_AudioControls` renders a capability badge above the
    PTT button: a globe icon for Global levels, a focus-frame
    icon for `L0Focused`, plus a Tooltip exposing the bound input
    class. New ARB key `pttCapabilityBadge(level, backend)` in
    `app_en.arb` and `app_zh.arb`.

Per-platform global PTT backends (`WindowsRawInputBackend`,
`MacOSEventTapBackend`, `LinuxGnomeWaylandBackend`) and the
`MissedKeyUpWatchdog` task land in a separate follow-up commit;
this milestone ships only PTT-L0 universally so the application's
runtime capability reporting is honest from day one.

Tests
-----

* `chanora_audio` rises from 1 to 4 unit tests covering
  `PttCapabilityLevel::as_str`, `is_global`, and the
  `PttBackendDescriptor::focused()` shape contract.
* `chanora_diagnostics` rises from 9 to 11 unit tests covering
  the new `PttBanCheckVisitor` over every banned field name and
  the `PTT_BANNED_FIELDS` stability assertion.
* Workspace total: 53 unit + integration tests, all green with
  `CHANORA_DISABLE_KEYRING=1` (was 49 at v1.0.0-rc.2).
* `flutter analyze`: clean.
* `cargo deny check`: advisories ok, bans ok, licenses ok,
  sources ok.
* `cargo about generate`: zero warnings (license inventory
  regenerated).
* `tools/dump_flutter_licenses.sh`: 94 packages, zero without
  LICENSE.
* Linux x86_64 release bundle builds clean.

No Android live verification in this commit per the user's note
that the test device was removed. Android arm64-v8a continues to
build via the same `cargo ndk` path; runtime reporting on Android
is `L0Focused` for the foreseeable future.

apps/chanora_flutter/lib/main.dart

@@ -107,6 +107,14 @@ class _BetaHomeState extends State<_BetaHome> {
   bool _outputMuted = false;
   double _outputGain = 1.0;
 
+  // Desktop PTT capability badge state (gen2 v0.9.3 / SDD-091).
+  // Populated by `BridgeEvent.PttCapability`. Defaults match the
+  // universal `FocusedPttBackend::focused()` descriptor so the UI
+  // shows an honest baseline even before the first event arrives.
+  String _pttLevel = 'L0Focused';
+  String _pttBackendId = 'focused';
+  String _pttBoundInputClass = 'keyboard';
+
   List<rust.BridgeBookmark> _bookmarks = const [];
 
   @override
@@ -160,6 +168,16 @@ class _BetaHomeState extends State<_BetaHome> {
         setState(() => _audioStarted = false);
       case rust.BridgeEvent_SnapshotChanged():
         unawaited(_onRefresh());
+      case rust.BridgeEvent_PttCapability(
+            :final level,
+            :final backendId,
+            :final boundInputClass,
+          ):
+        setState(() {
+          _pttLevel = level;
+          _pttBackendId = backendId;
+          _pttBoundInputClass = boundInputClass;
+        });
     }
   }
 
@@ -644,6 +662,9 @@ class _BetaHomeState extends State<_BetaHome> {
                   inputMuted: _inputMuted,
                   outputMuted: _outputMuted,
                   outputGain: _outputGain,
+                  pttLevel: _pttLevel,
+                  pttBackendId: _pttBackendId,
+                  pttBoundInputClass: _pttBoundInputClass,
                   onPttDown: () => _setPtt(true),
                   onPttUp: () => _setPtt(false),
                   onToggleInputMute: _toggleInputMute,
@@ -798,6 +819,9 @@ class _AudioControls extends StatefulWidget {
     required this.inputMuted,
     required this.outputMuted,
     required this.outputGain,
+    required this.pttLevel,
+    required this.pttBackendId,
+    required this.pttBoundInputClass,
     required this.onPttDown,
     required this.onPttUp,
     required this.onToggleInputMute,
@@ -809,6 +833,9 @@ class _AudioControls extends StatefulWidget {
   final bool inputMuted;
   final bool outputMuted;
   final double outputGain;
+  final String pttLevel;
+  final String pttBackendId;
+  final String pttBoundInputClass;
   final VoidCallback onPttDown;
   final VoidCallback onPttUp;
   final VoidCallback onToggleInputMute;
@@ -835,9 +862,44 @@ class _AudioControlsState extends State<_AudioControls> {
             stats.pttActive ? 'on' : 'off',
           );
 
+    final isGlobal = widget.pttLevel != 'L0Focused';
+    final badgeLabel = l10n.pttCapabilityBadge(
+      widget.pttLevel,
+      widget.pttBackendId,
+    );
+
     return Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
+        // Capability badge (gen2 v0.9.3 / SDD-091). Renders the
+        // active PTT level + backend so the user understands when
+        // Global PTT has fallen back to Focused PTT.
+        Padding(
+          padding: const EdgeInsets.only(bottom: 6),
+          child: Tooltip(
+            message: widget.pttBoundInputClass.isEmpty
+                ? badgeLabel
+                : '$badgeLabel\n(${widget.pttBoundInputClass})',
+            child: Row(
+              children: [
+                Icon(
+                  isGlobal ? Icons.public : Icons.crop_free,
+                  size: 14,
+                  color: theme.colorScheme.onSurfaceVariant,
+                ),
+                const SizedBox(width: 4),
+                Expanded(
+                  child: Text(
+                    badgeLabel,
+                    style: theme.textTheme.bodySmall?.copyWith(
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
         Listener(
           onPointerDown: (_) {
             setState(() => _pressed = true);