docs: log PR #13 (Mat4 canonical zero + MulAssign) in execution plan and decision log

Author: flyingrobots

docs/decision-log.md

@@ -17,3 +17,9 @@
 | 2025-10-27 | PR #7 prep | Extracted math + engine spike into `rmg-core` (split-core-math-engine); added inline rustdoc on canonical snapshot hashing (node/edge order, payload encoding). | Land the isolated, reviewable portion now; keep larger geometry/broad‑phase work split for follow-ups. | After docs update, run fmt/clippy/tests; merge is a fast‑forward over `origin/main`. |
 | 2025-10-28 | PR #7 finalize | Reachability-only snapshot hashing; ports demo registers rule; guarded ports footprint; scheduler `finalize_tx()` clears `pending`; `PortKey` u30 mask; hooks+CI hardened (toolchain pin, rustdoc fixes). | Determinism + memory hygiene; remove test footguns; pass CI with stable toolchain while keeping rmg-core MSRV=1.68. | Merge PR #7 after green CI; queue MWMR Phase 2 perf tests + retry policy work. |
 | 2025-10-27 | MWMR reserve gate | Engine calls `scheduler.finalize_tx()` at commit; compact rule id used on execute path; per‑tx telemetry summary behind feature. | Enforce independence and clear active frontier deterministically; keep ordering stable with `(scope_hash, family_id)`. | Toolchain pinned to Rust 1.68; add design note for telemetry graph snapshot replay. |
+## 2025-10-28 — Mat4 canonical zero + MulAssign (PR #13)
+
+- Decision: Normalize -0.0 from trig constructors in Mat4 and add MulAssign for in-place multiplication.
+- Rationale: Avoid bitwise drift in snapshot/matrix comparisons across platforms; improve ergonomics in hot loops.
+- Impact: No API breaks. New tests assert no -0.0 in rotation matrices at key angles; added `MulAssign` for owned/&rhs.
+- Next: Review feedback; if accepted, apply same canonicalization policy to other math where applicable.

docs/execution-plan.md

@@ -33,6 +33,11 @@ This is Codex’s working map for building Echo. Update it relentlessly—each s
 
 ## Today’s Intent
 
+> 2025-10-28 — PR #13 (math polish) opened
+
+- Focus: canonicalize -0.0 in Mat4 trig constructors and add MulAssign ergonomics.
+- Outcome: Opened PR echo/core-math-canonical-zero with tests; gather feedback before merge.
+
 > 2025-10-28 — PR #7 finalize and land
 
 - Focus: close out review nits on engine/scheduler/footprint/demo; ensure CI/hook stability; keep scope tight.