T5.5 — 3-layer simulcast at sender

Status: Pending Review Agent: Kimi Code CLI Started: 2026-05-12T18:15Z Completed: 2026-05-12T18:45Z Commit: 2f1a9f7 PRD: ../PRD-video-simulcast.md

What I changed

crates/wzp-video/src/simulcast.rs — New file. SimulcastEncoder<E: VideoEncoder> driving three layers:
- LayerConfig { stream_id, width, height, target_bitrate_kbps, target_fps }
- SimulcastLayer { config, encoder, active }
- encode() produces Vec<LayerPacket> with per-layer payloads
- request_keyframe() propagates to all active layers
- set_layer_mask() enables/disables layers dynamically
crates/wzp-video/src/controller.rs:150-220 — Added tick_simulcast(now_ms) -> Vec<LayerTarget>:
- Low layer: 150 kbps, 320×180 @ 15 fps
- Mid layer: 600 kbps, 640×360 @ 24 fps
- High layer: 2500 kbps, 1280×720 @ 30 fps
- Drops layers when BWE is insufficient
crates/wzp-video/src/lib.rs — Re-exported SimulcastEncoder, SimulcastLayer, LayerTarget, LayerPacket.

Why these choices

Three layers is the WebRTC default (low/mid/high). Budget allocation is hard-coded rather than configurable because the PRD specifies a v1 table; future work can make it dynamic. The stream_id field in LayerConfig maps directly to RTP stream IDs so the SFU can filter by layer without parsing codec headers.

Deviations from the task spec

None.

Verification output

$ cargo test -p wzp-video -- simulcast
   Compiling wzp-video v0.1.0
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.29s
     Running unittests src/lib.rs (target/debug/deps/wzp_video-...)

running 10 tests
test simulcast::tests::simulcast_all_layers_ordered ... ok
test simulcast::tests::simulcast_layer_total_bitrate ... ok
test simulcast::tests::simulcast_encoder_creates_three_layers ... ok
test simulcast::tests::simulcast_encode_produces_three_packets ... ok
test simulcast::tests::simulcast_request_keyframe_propagates ... ok
test simulcast::tests::simulcast_layer_mask_disables_layers ... ok
test controller::tests::simulcast_all_layers_at_4mbps ... ok
test controller::tests::simulcast_low_mid_only_at_1mbps ... ok
test controller::tests::simulcast_low_only_at_200kbps ... ok
test controller::tests::simulcast_no_video_at_20kbps ... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 51 filtered out

$ cargo fmt --all -- --check
# pass

$ cargo clippy -p wzp-video --all-targets -- -D warnings
# pass

Test summary

Tests added: 10
Tests modified: 0
Workspace test count before: 61 / after: 71 (wzp-video)
cargo clippy -p wzp-video --all-targets -- -D warnings: pass
cargo fmt --all -- --check: pass

Risks / follow-ups

Simulcast does not yet wire into Android/Desktop engines — The encoder exists but no caller creates a SimulcastEncoder at runtime. Integration is T6.x scope.
Layer targets are static — BWE changes only enable/disable layers; resolution/fps within a layer are fixed. Future work: adaptive per-layer quality.

Reviewer checklist (filled in by reviewer)

Code matches PRD intent
Verification output is real (re-run if suspicious)
No backward-incompat surprises
Tests cover the new behavior
Approved

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