docs(ptt): land Baseline Candidate v0.9.3 — capability-based desktop PTT

Applies the gen2 desktop-PTT review summary
(`gen2/chanora-desktop-ptt-review-summary-v0.9.2.md`) to our doc set
with the owner rulings PTT-OPEN-001 through PTT-OPEN-006 resolved as
accepted decisions DEC-023 through DEC-028:

  * DEC-023 Windows Global PTT P0 / MVP
  * DEC-024 macOS Global PTT P0 / MVP with permission UX
  * DEC-025 Linux officially-tested env: GNOME on Wayland only
  * DEC-026 Mouse side buttons supported (Win + macOS; Linux portal)
  * DEC-027 PTT diagnostics: capability + availability only, no
            raw key codes ever
  * DEC-028 Missed-key-up watchdog: P0

Requirements (SysRS / SRS) and architecture (SysDes / SAD / SDD)
gain the desktop-PTT ID set the gen2 summary describes:

  SysRS-296..302  -> SysDes-142..148
                  -> SRS-195..203
                  -> SAD-071..079
                  -> SDD-081..092

ID totals advance from 295 / 141 / 194 / 70 / 80 to 302 / 148 / 203
/ 79 / 92. The strict layered sourcing rule (`SRS -> SysDes` only,
`SAD -> SRS` only, `SDD -> SAD` only) is preserved; the
`tools/validate_docs.py` validator reports zero undefined refs and
zero direct-layer-rule violations.

New document:

  * `docs/architecture/desktop-ptt-architecture.md` — capability
    ladder (L0Focused, L1GlobalShortcut, L2GlobalHoldToTalk,
    L3GlobalWithMouseButtons, L4DeviceAware reserved), Windows /
    macOS / Linux strategies, privacy rule, audio-gate rule,
    missed-key-up watchdog, release-readiness evidence requirement,
    traceability summary.

Doc addenda (Baseline Candidate 0.9.3):

  * `privacy/privacy-policy.md` — no raw key history, capability-
    dependent Global PTT, UI reflects actual runtime capability
  * `security/threat-model.md` — THREAT-PTT-001..006
  * `security/diagnostic-redaction-audit-report.md` —
    REDACT-PTT-001..006 banned field list enforced by `PttSanitizer`
  * `release/platform-release-policy.md` — per-platform evidence
    fields, no over-claim on untested Linux compositors
  * `release/release-readiness-go-nogo-record.md` — RR-PTT-001..008
    release-readiness items
  * `verification/swe4-unit-verification-plan.md` —
    SWE4-UV-035..039
  * `verification/swe5-software-integration-verification-plan.md` —
    SWE5-IV-015
  * `verification/swe6-software-verification-plan.md` — SWE6-SV-017
  * `verification/sys4-system-integration-verification-plan.md` —
    SYS4-SIV-016
  * `governance/traceability-matrix.md` — full PTT trace rows +
    verification map
  * `governance/decision-impact-assessment.md` — DEC-023..028
    impact matrix
  * `governance/product-decision-register.md` v0.9.9 entry
    recording DEC-023..028 in the decision table and the status
    table at §7
  * `governance/document-index.md` — adds
    `desktop-ptt-architecture.md` to the controlled set
  * `architecture/proof-of-concept-plan.md` —
    PoC-PTT-001..005 platform items
  * `references/external-references.md` — Windows Raw Input,
    macOS event-tap, Linux GlobalShortcuts portal references
  * Both validation reports
    (`baseline-candidate-validation-report.md`,
    `repo-format-validation-report.md`) bumped to v0.9.3 with the
    new ID totals (302 / 148 / 203 / 79 / 92).

README §"Desktop Push-to-Talk" added between Architecture Overview
and Repository Layout: capability levels, per-platform strategy,
privacy posture, missed-key-up watchdog.

Tooling:

  * `tools/validate_docs.py` copied from the gen2 zip into the
    repo tree (was previously available only inside the zip).
    Reports zero undefined refs, zero direct-layer-rule violations,
    English-only CJK check passes. The 35 "old package-style
    filename" hits are pre-existing and identical to the gen2
    baseline (they live in `path-migration-map.md` and config-ID
    headers of governance docs and are intentional per the path
    migration policy).
  * `.gitignore` adds `/gen2/` so the externally-provided review
    package does not enter the repo.

No code changes in this commit; B (the implementation split into
`transmit_active` / `capture_active`, `PttCapabilityLevel`
reporting, `PttSanitizer` diagnostics rule, and the UI capability
badge) follows in a separate commit.

README.md

@@ -113,6 +113,46 @@ The current recommended MVP scope is:
 
 ---
 
+## Desktop Push-to-Talk
+
+Chanora's desktop Push-to-Talk (PTT) follows a **capability-based** design (see
+[`docs/architecture/desktop-ptt-architecture.md`](docs/architecture/desktop-ptt-architecture.md)).
+Focused PTT — the user holds a bound key or mouse button inside the
+focused Chanora window — is mandatory on Windows, macOS, and Linux.
+Global PTT (recognised while the application is not focused) is
+**capability-dependent**: it requires the operating system, the
+user-granted permission set, the display server, and the available
+input backend to all permit it.
+
+The application reports a `PttCapabilityLevel` (`L0Focused`,
+`L1GlobalShortcut`, `L2GlobalHoldToTalk`, `L3GlobalWithMouseButtons`)
+that matches actual runtime behaviour, not the platform's theoretical
+maximum. The UI capability badge shows the live value.
+
+Per-platform strategy (resolved by owner rulings 2026-05-15, see
+`docs/governance/product-decision-register.md` DEC-023 through
+DEC-028):
+
+* **Windows** — Raw Input first, low-level keyboard hook fallback,
+  Focused PTT terminal fallback. Mouse side buttons supported.
+  P0 / MVP.
+* **macOS** — permission-aware Event Tap with Focused PTT fallback;
+  Global PTT upgrades asynchronously when the user grants Input
+  Monitoring / Accessibility. P0 / MVP.
+* **Linux** — officially tested on **GNOME on Wayland** using the
+  `org.freedesktop.portal.GlobalShortcuts` interface; every other
+  Linux environment falls back to Focused PTT. Release notes do
+  not claim Global PTT support outside the tested compositor.
+* Raw key codes, scan codes, virtual-key values, keysyms, and
+  key-press timing sequences are never logged or included in the
+  user-initiated diagnostic export. The diagnostic export carries
+  only capability level, backend identifier, and bound input
+  class.
+
+A missed-key-up watchdog (default 30 s) clears `transmit_active`
+when the OS suppresses a key-up event so a stuck-PTT bug class is
+ruled out by construction.
+
 ## Repository Layout
 
 The repository documentation is expected to live under `docs/`.