feat(go): fixture capture + characterization framework (M3)

Closes M3.1–M3.6.  Parity safety net proving Go output matches Python
for every ported pure-domain function (M2.1–M2.9) and reconcile (M2.10).

Capture pipeline:
- scripts/capture_fixtures.py: calls each Python function with seeded
  inputs, emits JSON fixtures to stdout (never writes files directly).
- scripts/scrub_fixtures.py: deterministic PII scrubber — SHA-256
  pseudonyms for member names, digit-preserving hashes for VS/account/
  bank_id, name-sweep in message text.  Idempotent; no salt.
- scripts/_fixture_seeds.py: handcrafted seeds for all 11 functions;
  synthetic names throughout (no real roster members).
- scripts/capture_all_fixtures.sh: convenience wrapper for full corpus
  regeneration outside of make.

Fixture corpus (98 files, all PII-free):
- go/tests/fixtures/pure/<func>/<case>.json — 10 function directories.
- go/tests/fixtures/reconcile/<NN>_<case>.json — 10 branch-coverage
  cases: greedy, overpayment credit, proportional remainder, even-split,
  out-of-window, exception override, other: purpose, junior ?, multi-
  person+month fan-out, unmatched.

Go parity tests (//go:build parity):
- go/tests/parity/parityio.go: generic LoadDir/RunAll helpers + typed
  In/Out struct pairs for all 10 pure functions; Envelope decoder for
  int/float/none disambiguation.
- 10 pure-function test packages + bespoke reconcile test with per-cell
  float tolerance (math.Abs <= 0.01 for `paid` values).

Makefile: go-parity, go-test-all, capture-fixtures targets.
go/tests/fixtures/README.md: refresh workflow + PII audit guide.

Gate: make go-test green, make go-parity green (11/11 packages),
      make go-lint clean (parity tag), make go-build clean.

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

CHANGELOG.md

@@ -1,5 +1,16 @@
 # Changelog
 
+## 2026-05-06 23:25 CEST — feat(go/M3): fixture capture + parity test framework
+
+- `scripts/capture_fixtures.py`: dispatcher CLI that calls each ported function with seeded inputs and emits captured output as JSON fixtures.
+- `scripts/scrub_fixtures.py`: deterministic PII scrubber (SHA-256 pseudonyms, digit-preserving account/VS hashes, name-sweep in free text).
+- `scripts/_fixture_seeds.py`: handcrafted seed registry for all 10 pure functions + 10 reconcile branch-coverage cases.
+- 98 fixture files committed under `go/tests/fixtures/pure/<func>/` and `go/tests/fixtures/reconcile/`; all PII-free.
+- `go/tests/parity/parityio.go`: shared loader with generic `LoadDir`/`RunAll` helpers and typed `In`/`Out` structs for all 10 functions.
+- 11 parity test packages under `//go:build parity`: 10 pure-function tests + bespoke reconcile test with per-cell float tolerance.
+- Makefile: `go-parity`, `go-test-all`, `capture-fixtures` targets.
+- `go/tests/fixtures/README.md`: refresh workflow, PII audit guide, adding-a-fixture steps.
+
 ## 2026-05-06 17:49 CEST — feat(go/M2.11-12): wire fuj fees + fuj reconcile subcommands
 
 - New `go/internal/services/membership` package: `AttendanceLoader`, `TransactionLoader`, `ExceptionLoader` interfaces, a stub (`NewStubSources`) that returns `ErrIOPending`, and `FeesReport` / `ReconcileReport` orchestration functions backed by real `domain/fees` + `domain/reconcile` logic.