[AlxcBTW]

Gap 1 of T38: the stream no longer keeps the frame ring's size for the whole share.

• The host's hub applies the controller's whole target: bitrate and parity as before, and now size and frame rate as a new epoch (Start to the worker, config to the guest, key frame).
• The frame ring is made at the top rung (1080p60); the worker's encoders already scale each picture to the stream's size.
• RateController::set_max_height (the player's limit for share_quality, wired in the next PR).
• On the pause rung version 0 keeps a 540p30 picture at 300 kbit/s instead of stopping it (noted in the spec).

Tests: retune unit tests (same target, rate only, shape change, epoch wrap, pause), controller tests for the height limit, and an in-process integration test in crates/net/tests/play.rs (heavy loss reported by the guest moves host and guest to 540p30 in a new epoch and whole pictures keep coming).

Not proven: a real game's ring at 1080p, real encoders' reaction to a size change on real hardware.

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