chore: tick M3.1–M3.6 in progress tracker + CHANGELOG entry

Merges SHA 57518a8 (PR #12).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md

@@ -65,14 +65,14 @@ Each task: port the function, write Go unit tests for fresh cases, hook into the
 
 Goal: deterministic, PII-free fixture corpus that drives parity tests. Runs in parallel with M2 (M3.1/M3.2 unblocks M2.1).
 
-- [x] **M3.1** `scripts/capture_fixtures.py` — pure-function output dumper. Reads inputs from stdin / argv, prints `{"input":..., "output":...}` JSON
-- [x] **M3.2** `scripts/scrub_fixtures.py` — replaces names with `Member_<8hex>` (deterministic per name); scrambles sender/account/VS/bank_id with stable bijection; preserves dates, amounts, exception keys
-- [x] **M3.3** Capture pure-fn fixtures for M2.1–M2.9 (run helper + scrubber, commit to `tests/fixtures/pure/<func>/<case>.json`)
-- [x] **M3.4** Capture ~10 reconcile fixtures spanning every code path: greedy, proportional (float remainder), even-split, out-of-window credit, exception override, `other:` purpose, junior `"?"`, multi-person comma-split, multi-month range, unmatched. Commit to `tests/fixtures/reconcile/`
-- [x] **M3.5** Hook fixtures into Tier-1 test runner with `-tags=parity` build constraint
-- [x] **M3.6** Document fixture-refresh workflow in `tests/fixtures/README.md` (what to do when sheet schema changes)
+- [x] **M3.1** `scripts/capture_fixtures.py` — pure-function output dumper. Reads inputs from stdin / argv, prints `{"input":..., "output":...}` JSON — `57518a8`
+- [x] **M3.2** `scripts/scrub_fixtures.py` — replaces names with `Member_<8hex>` (deterministic per name); scrambles sender/account/VS/bank_id with stable bijection; preserves dates, amounts, exception keys — `57518a8`
+- [x] **M3.3** Capture pure-fn fixtures for M2.1–M2.9 (run helper + scrubber, commit to `tests/fixtures/pure/<func>/<case>.json`) — `57518a8`
+- [x] **M3.4** Capture ~10 reconcile fixtures spanning every code path: greedy, proportional (float remainder), even-split, out-of-window credit, exception override, `other:` purpose, junior `"?"`, multi-person comma-split, multi-month range, unmatched. Commit to `tests/fixtures/reconcile/` — `57518a8`
+- [x] **M3.5** Hook fixtures into Tier-1 test runner with `-tags=parity` build constraint — `57518a8`
+- [x] **M3.6** Document fixture-refresh workflow in `tests/fixtures/README.md` (what to do when sheet schema changes) — `57518a8`
 
-**Gate:** ✅ `tests/fixtures/` populated (98 files); `make go-parity` green; `make go-lint` (parity tag) clean; raw `tmp/*.json` confirmed gitignored.
+**Gate:** ✅ `tests/fixtures/` populated (98 files); `make go-parity` green; `make go-lint` (parity tag) clean; raw `tmp/*.json` confirmed gitignored. Merged as `57518a8`.
 
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