feat(go): M5.4 — parity diff binary + make parity

Adds cmd/parity/main.go: a standalone Go binary that GETs
/api/version, /api/adults, /api/juniors, /api/payments from both
the Python (:5001) and Go (:8080) backends, scrubs an allowlist
(render_time.total, build_meta), 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.

- go/cmd/parity/main.go: flags (-py, -go, -route, -timeout), fetch
  helper, allowlist scrubber (dotted-path aware), exit-code logic.
- go/cmd/parity/scrub_test.go: 4 unit tests for the scrubber.
- go/go.mod: promote github.com/google/go-cmp to direct dep.
- Makefile: parity target + help entry.
- Progress tracker: M5.4 ticked; milestone updated to M5 complete.
- Plan archived to docs/plans/2026-05-07-2254-m5-4-parity-binary.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help fees match web web-py web-debug web-go go-build go-test go-test-all go-parity go-run go-sync-debug go-lint capture-fixtures image run sync sync-2026 test test-v docs
+.PHONY: help fees match web web-py web-debug web-go go-build go-test go-test-all go-parity go-run go-sync-debug go-lint capture-fixtures parity image run sync sync-2026 test test-v docs
 
 export PYTHONPATH := scripts:$(PYTHONPATH)
 VENV := .venv
@@ -29,6 +29,7 @@ help:
 	@echo "  make go-lint     - Run golangci-lint on Go code"
 	@echo "  make go-sync-debug [DAYS=N] - Dry-run Go sync with Fio debug logs and txn table (default DAYS=30)"
 	@echo "  make capture-fixtures - Regenerate parity fixture corpus from live Python"
+	@echo "  make parity      - Diff /api/* between web-py (:5001) and web-go (:8080); both must be running"
 	@echo "  make image       - Build Python OCI container image"
 	@echo "  make run         - Run the built Python Docker image locally"
 	@echo "  make sync        - Sync Fio transactions to Google Sheets"
@@ -102,6 +103,9 @@ go-lint:
 web-go: go-build
 	./$(GO_BIN) server
 
+parity:
+	cd $(GO_SRC) && go run ./cmd/parity $(ARGS)
+
 image:
 	docker build -t fuj-management:latest \
 		--build-arg GIT_TAG=$$(git describe --tags --always 2>/dev/null || echo "untagged") \