Merge pull request 'feat(go): M5.4 — parity diff binary + make parity' (#19) from feat/go-m5-4-parity-binary into main

Reviewed-on: #19

CHANGELOG.md

@@ -12,6 +12,14 @@
 - Why: both backends used `tmp/<key>_cache.json` with the same keys (`attendance_regular`, `attendance_juniors`, `payments_transactions`, `exceptions_dict`) but different shapes — Python caches post-processed view-model tuples, Go caches raw rows. Whichever wrote last poisoned the cache; running both in parallel produced `ValueError: too many values to unpack (expected 2, got 68)` on Python's `/adults` after the Go server populated `attendance_regular_cache.json` with raw CSV rows.
 - After upgrading: stop the Go server, hit `/flush-cache` on the Python side once (rewrites `tmp/*.json` with correct shapes), then restart `make web-go` — it will use `tmp/go/` going forward. Required for the M5.4 `make parity` workflow which assumes both backends run side-by-side.
 
+## 2026-05-07 22:55 CEST — feat(go): M5.4 — parity diff binary + `make parity`
+
+- `go/cmd/parity/main.go`: new standalone binary that GETs `/api/adults`, `/api/juniors`, `/api/payments` from both Python (:5001) and Go (:8080) backends, scrubs an allowlist (`render_time.total`), and prints `cmp.Diff` for any remaining differences. Exits 0 on full match, 1 on diffs, 2 on fetch/parse errors — CI-friendly for M7.2. `/api/version` is excluded by design (returns binary identity — tag/commit/build_date — which differs between independently built backends); still accessible via `make parity ARGS="-route /api/version"`.
+- `go/cmd/parity/scrub_test.go`: 4 unit tests covering top-level delete, nested delete, missing path, and non-map parent.
+- `go/go.mod`: `github.com/google/go-cmp` promoted to direct dependency.
+- `Makefile`: `parity` target added (`.PHONY`, help, `cd go && go run ./cmd/parity`).
+- `docs/plans/2026-05-07-2254-m5-4-parity-binary.md`: plan archived.
+
 ## 2026-05-07 22:37 CEST — feat(py): M5.3 — Python /api/* shadow endpoints
 
 - `app.py`: four new JSON routes (`/api/version`, `/api/adults`, `/api/juniors`, `/api/payments`) mirroring the Go `/api/*` handlers; `_unwrap_view_model_for_api()` helper expands pre-serialised JSON strings and renames `month_labels_json` → `month_labels`, `raw_payments_json` → `raw_payments` to match Go wire contract.