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>

go/go.mod

@@ -3,6 +3,7 @@ module fuj-management/go
 go 1.26.1
 
 require (
+	github.com/google/go-cmp v0.7.0
 	github.com/invopop/jsonschema v0.14.0
 	golang.org/x/net v0.53.0
 	golang.org/x/text v0.36.0