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>

go/tests/fixtures/pure/infer_transaction_details/member_in_message.json

@@ -0,0 +1,36 @@
+{
+  "case": "member_in_message",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-05-06",
+  "input": {
+    "tx": {
+      "sender": "Test Payer",
+      "message": "alzbeta testovicka leden 2026",
+      "user_id": "",
+      "date": {
+        "type": "string",
+        "value": "2026-01-15"
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Alžběta Testovická",
+        "confidence": "auto"
+      }
+    ],
+    "months": [
+      "2026-01"
+    ],
+    "search_text": "Test Payer alzbeta testovicka leden 2026 "
+  }
+}

go/tests/fixtures/pure/infer_transaction_details/member_in_sender.json

@@ -0,0 +1,36 @@
+{
+  "case": "member_in_sender",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-05-06",
+  "input": {
+    "tx": {
+      "sender": "Tomáš Fiktivný",
+      "message": "FUJ trenink",
+      "user_id": "",
+      "date": {
+        "type": "string",
+        "value": "2026-02-01"
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Tomáš Fiktivný (Tov)",
+        "confidence": "auto"
+      }
+    ],
+    "months": [
+      "2026-02"
+    ],
+    "search_text": "Tomáš Fiktivný FUJ trenink "
+  }
+}

go/tests/fixtures/pure/infer_transaction_details/month_fallback_from_date.json

@@ -0,0 +1,36 @@
+{
+  "case": "month_fallback_from_date",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-05-06",
+  "input": {
+    "tx": {
+      "sender": "Alžběta Testovická",
+      "message": "platba",
+      "user_id": "",
+      "date": {
+        "type": "string",
+        "value": "2026-03-15"
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Alžběta Testovická",
+        "confidence": "auto"
+      }
+    ],
+    "months": [
+      "2026-03"
+    ],
+    "search_text": "Alžběta Testovická platba "
+  }
+}

go/tests/fixtures/pure/infer_transaction_details/no_member_no_month.json

@@ -0,0 +1,28 @@
+{
+  "case": "no_member_no_month",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-05-06",
+  "input": {
+    "tx": {
+      "sender": "Unknown Person",
+      "message": "random text",
+      "user_id": "",
+      "date": {
+        "type": "none"
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [],
+    "months": [],
+    "search_text": "Unknown Person random text "
+  }
+}

go/tests/fixtures/pure/infer_transaction_details/serial_date.json

@@ -0,0 +1,36 @@
+{
+  "case": "serial_date",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-05-06",
+  "input": {
+    "tx": {
+      "sender": "Jana Nováková",
+      "message": "leden",
+      "user_id": "",
+      "date": {
+        "type": "float",
+        "value": 46027.0
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Jana Nováková",
+        "confidence": "auto"
+      }
+    ],
+    "months": [
+      "2026-01"
+    ],
+    "search_text": "Jana Nováková leden "
+  }
+}