T2.1: Add SignalMessage::TransportFeedback

docs/PRD/TASKS.md

@@ -1312,8 +1312,8 @@ Statuses (in order of progression):
 | T1.5.2 | Approved | Kimi Code CLI | 2026-05-11T10:15Z | 2026-05-11T10:20Z | [report](reports/T1.5.2-report.md) | Approved. PROTOCOL-AUDIT.md known-debt section present; standard #3 amended; report template updated. |
 | T1.6 | Approved | Kimi Code CLI | 2026-05-11T10:20Z | 2026-05-11T11:05Z | [report](reports/T1.6-report.md) | Approved. Clean impl, both sides tested, T1.5 gap-fixes folded in with explicit disclosure — good course-correction from the T1.5 scope-creep review. |
 | T1.7 | Approved | Kimi Code CLI | 2026-05-11T11:05Z | 2026-05-11T16:29Z | [report](reports/T1.7-report.md) | Approved. W5 invariant already encoded in `to_bytes()` order; regression test pins it. Guards future encryption wiring. |
-| T1.8 | Pending Review | Kimi Code CLI | 2026-05-11T16:41Z | 2026-05-11T16:59Z | [report](reports/T1.8-report.md) | — |
-| T2.1 | Open | — | — | — | — | — |
+| T1.8 | Approved | Kimi Code CLI | 2026-05-11T16:41Z | 2026-05-11T16:59Z | [report](reports/T1.8-report.md) | Approved. Per-stream/per-MediaType windows; AEAD-first then anti-replay; plaintext rollback on detection. W11 resolved. |
+| T2.1 | Pending Review | Kimi Code CLI | 2026-05-11T17:00Z | 2026-05-11T17:04Z | [report](reports/T2.1-report.md) | — |
 | T2.2 | Open | — | — | — | — | — |
 | T2.3 | Open | — | — | — | — | — |
 | T2.4 | Open | — | — | — | — | — |
@@ -1348,5 +1348,6 @@ Statuses (in order of progression):
 Items currently waiting on the reviewer:
 
 - T1.8 — Per-stream anti-replay window with configurable size — report: reports/T1.8-report.md
+- T2.1 — Add `SignalMessage::TransportFeedback` — report: reports/T2.1-report.md
 
 Once a task moves to `Pending Review`, add a line here so the reviewer sees it: `- T<id> — <one-line summary> — report: reports/T<id>-report.md`. The reviewer removes the line when they mark it `Approved` (or moves it back to the agent on `Changes Requested`).

docs/PRD/reports/T2.1-report.md

@@ -0,0 +1,64 @@
+# 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