Add USB tether transport with source-timed A/V

[YashasVM]

What changed

• adds an Android USB tether/RNDIS transport mode using the existing single MediaCodec + AAC + MPEG-TS/SRT pipeline
• adds Windows USB tether gateway detection and a 30 ms wired SRT profile in the OBS source
• preserves decoded MPEG-TS source timestamps on one shared A/V timeline instead of replacing them with arrival time
• bounds camera-control work at 16 queued commands with reject-newest overflow behavior
• makes discovery restart generation-safe and makes USB startup wait for cable/tether availability
• documents USB setup and physical acceptance measurements

Why

USB should provide a lower-jitter, lower-latency link without adding another phone encoder or re-encoding ISO video. The shared timestamp fix also addresses the existing A/V drift risk on both Wi-Fi and USB.

Validation

• python -m pytest -q — 30 passed
• Android :app:testDebugUnitTest :app:lintDebug :app:assembleDebug — passed, including native builds for arm64-v8a, armeabi-v7a, x86, and x86_64
• pinned OBS 32.2.1 / FFmpeg 62 / Qt 6.8.3 plugin build — passed
• ctest --test-dir obs-plugin/build -C Release --output-on-failure — native media clock test passed

Before merge

This stays draft until a physical Android phone/cable run records Wi-Fi-vs-USB p50/p95 latency, maximum A/V skew, a 30-minute 1080p60 soak, and five cable unplug/replug recoveries as required by the repository performance policy.

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