T1.6: Protocol version negotiation in handshake

docs/PRD/reports/T1.5.1-report.md

@@ -1,6 +1,6 @@
 # T1.5.1 — Remove `unwrap()` from `encode_compact`
 
-**Status:** Pending Review
+**Status:** Approved
 **Agent:** Kimi Code CLI
 **Started:** 2026-05-11T10:09Z
 **Completed:** 2026-05-11T10:15Z
@@ -58,8 +58,12 @@ None.
 
 ## 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
+- [x] Code matches PRD intent — unwrap replaced with `if let Some(base)`, falls through to full-frame on missing baseline
+- [x] Verification output is real — re-ran `cargo test -p wzp-proto encode_compact` (passes), confirmed only test-code unwraps remain in `packet.rs`
+- [x] No backward-incompat surprises — public signature of `encode_compact` unchanged
+- [x] Tests cover the new behavior — `encode_compact_fallback_to_full_without_baseline` is the right shape
+- [x] Approved
+
+### Reviewer notes (2026-05-11)
+
+Approved. Clean fix. The Cargo.lock delta (1054 lines) is patch-version churn from `cargo build` resolving newer compatible deps (bitflags 2.11.0→2.11.1, aws-lc-rs 1.16.2→1.16.3, etc.) — legitimate per standard #8. Worth disclosing in "What I changed" next time, but it's a real build artifact not a hand edit.