fix(audio): bound playout depth to stop voice latency ratcheting up

Latency between speakers grew to multiple seconds and only reset on
rejoining voice. Root cause was the receiver playout logic, not the
codec settings: the playout clock free-ran in real time while the
sender omitted silence from its timestamps (and set no header flags),
and the only correction snapped the clock to the *oldest* buffered
frame — which could only ever add standing latency. target_depth_ms_
was computed but never enforced, so latency could only grow or reset.

Fix: bound playout against the stream's leading edge (newest frame).
(Re)seed to the leading edge on start/marker/starve (no prebuffer, so
latency stays low), and frame-skip catch-up trims any backlog beyond
target+hysteresis — the missing downward force.

Hardening: sender now stamps kFlagMarker (talkspurt start) and kFlagDtx,
consumed on recv for clean resync; adaptive late-drop window; EWMA
outlier rejection so silence gaps/stragglers don't poison the estimate;
duplicate counting and ring-underrun diagnostics.

New test_jitter_depth asserts depth stays bounded (<200ms) while
arrivals outrun playback. ctest --preset dev green (27/27).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -8,7 +8,7 @@ and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](
 > server-mute, channel CRUD, in-app account management, disconnect/keepalive/reaper. Windows
 > WinForms C# client shipped (M4). **macOS AppKit client shipped** — `VoiceCatMac.xcodeproj`
 > at `clients/apple/macOS/`. **iOS SwiftUI client shipped** — `VoiceCatiOS.xcodeproj` at
-> `clients/apple/iOS/`. `ctest --preset dev` green — 26/26 tests.
+> `clients/apple/iOS/`. `ctest --preset dev` green — 27/27 tests.
 > External PCM feed/tap API (`vc_stream_feed_pcm` + `vc_set_pcm_sink`) shipped.
 > **Screen-audio sharing shipped on macOS (ScreenCaptureKit) and iOS (ReplayKit Broadcast
 > Upload Extension → host App Group ring → `vc_stream_feed_pcm`).** See [`PROGRESS.md`](PROGRESS.md).