# T2.1 — Add `SignalMessage::TransportFeedback`

**Status:** Pending Review
**Agent:** Kimi Code CLI
**Started:** 2026-05-11T17:00Z
**Completed:** 2026-05-11T17:04Z
**Commit:** (see git log)
**PRD:** ../PRD-transport-feedback-bwe.md

## What I changed

- `crates/wzp-proto/src/packet.rs` — Added `TransportFeedback` variant to `SignalMessage`:
  ```rust
  TransportFeedback {
      #[serde(default)] version: u8,
      stream_id: u8,
      acked_seqs: Vec<u32>,
      nacked_seqs: Vec<u32>,
      remb_bps: u32,
      recv_time_us: u64,
  }
  ```
- `crates/wzp-proto/Cargo.toml` — Added `bincode = "1"` to `[dev-dependencies]` for forward-compat serialization tests.

## Why these choices

`#[serde(default)]` on `version` ensures old senders that omit the field deserialize cleanly (version = 0). `bincode` is already used elsewhere in the workspace (e.g., `wzp-crypto` tests), so adding it as a dev-dependency carries no supply-chain risk.

## Deviations from the task spec

None.

## Verification output

```bash
$ cargo test -p wzp-proto transport_feedback
running 2 tests
test packet::tests::transport_feedback_roundtrip ... ok
test packet::tests::transport_feedback_default_version ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 113 filtered out; finished in 0.00s
```

## Test summary

- Tests added: 2
  - `transport_feedback_roundtrip` — JSON + bincode serialization/deserialization
  - `transport_feedback_default_version` — verifies omitted `version` field defaults to 0
- Tests modified: 0
- `wzp-proto` test count: 115 (was 113 before T2.1)
- `cargo clippy -p wzp-proto --all-targets -- -D warnings`: pass
- `cargo fmt --all -- --check`: pass

## Risks / follow-ups

- No production code consumes `TransportFeedback` yet — T2.2/T2.3 will wire the BWE layer to produce and ingest it.

## 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