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/match_members/common_surname_no_match.json

@@ -0,0 +1,18 @@
+{
+  "case": "common_surname_no_match",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "novak leden",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": []
+  }
+}

go/tests/fixtures/pure/match_members/exact_full_name.json

@@ -0,0 +1,23 @@
+{
+  "case": "exact_full_name",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "platba od alzbeta testovicka leden",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Alžběta Testovická",
+        "confidence": "auto"
+      }
+    ]
+  }
+}

go/tests/fixtures/pure/match_members/first_and_last.json

@@ -0,0 +1,23 @@
+{
+  "case": "first_and_last",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "jan nový payment tomas fiktivny",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Tomáš Fiktivný (Tov)",
+        "confidence": "auto"
+      }
+    ]
+  }
+}

go/tests/fixtures/pure/match_members/nickname_match.json

@@ -0,0 +1,23 @@
+{
+  "case": "nickname_match",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "payment from strudl",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Pavel Smutný (Štrúdl)",
+        "confidence": "auto"
+      }
+    ]
+  }
+}

go/tests/fixtures/pure/match_members/no_match.json

@@ -0,0 +1,18 @@
+{
+  "case": "no_match",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "xyz platba",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": []
+  }
+}

go/tests/fixtures/pure/match_members/review_lastname_only.json

@@ -0,0 +1,23 @@
+{
+  "case": "review_lastname_only",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "testovicka leden",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Alžběta Testovická",
+        "confidence": "review"
+      }
+    ]
+  }
+}

go/tests/fixtures/pure/match_members/two_members_exact.json

@@ -0,0 +1,27 @@
+{
+  "case": "two_members_exact",
+  "func": "scripts.match_payments.match_members",
+  "captured_at": "2026-05-06",
+  "input": {
+    "text": "pavel smutny a alzbeta testovicka",
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ]
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Alžběta Testovická",
+        "confidence": "auto"
+      },
+      {
+        "name": "Pavel Smutný (Štrúdl)",
+        "confidence": "auto"
+      }
+    ]
+  }
+}