feat(ios): live AVC bitrate control and keyframe requests

[gmegidish]

Summary

• SetAvcBitrate/RequestAvcKeyFrame previously rejected iOS devices (a leftover from the MJPEG days), leaving the adaptive-bitrate feedback loop dead on iOS: the device encoder ran uncapped for the entire session — observed 22–38 Mbps for a 602×1310 stream, saturating the path (RTT 180–480ms with spikes past 1.4s).
• Both platforms now accept the same JSON-RPC payload (screencapture.setBitrate with bps, screencapture.requestKeyFrame); only the transport differs — Android over the localabstract control socket, iOS as length-prefixed JSON written on the live H.264 stream connection.
• The control write must ride that exact conn: the broadcast extension's TCPServer redirects video output to its newest client, so a separate control connection would steal the stream. The conn is stashed on IOSDevice when the AVC capture dials in and cleared when the stream ends.
• Adds tests covering the wire framing (4-byte big-endian length + JSON), the Android-compatible payload, and the no-active-stream error.

Stacked on #357 (pointer receivers) — the conn stash requires *IOSDevice instances in the device cache. Device side: mobile-next/devicekit-ios-h264#5.

[coderabbitai]

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