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>

scripts/capture_all_fixtures.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Regenerate the full fixture corpus.
+# Safe to re-run — always overwrites.
+# Requires: tmp/*_cache.json present (for real-data seeds for parse_month_references and match_members).
+
+set -euo pipefail
+
+REPO="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+FIXTURES="$REPO/go/tests/fixtures"
+CAPTURE_CMD="PYTHONPATH=$REPO/scripts:. python3 $REPO/scripts/capture_fixtures.py"
+SCRUB_CMD="python3 $REPO/scripts/scrub_fixtures.py"
+
+run_func() {
+    local func="$1"
+    local dir="$FIXTURES/pure/$func"
+    mkdir -p "$dir"
+    echo "  Capturing $func..."
+    eval "$CAPTURE_CMD --func $func --all" | while IFS= read -r line; do
+        case_id="$(python3 -c "import sys,json; print(json.loads('''$line''')['case'])" 2>/dev/null || \
+                   echo "$line" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d['case'])")"
+        echo "$line" | python3 "$REPO/scripts/scrub_fixtures.py" > "$dir/${case_id}.json"
+    done
+}
+
+echo "==> Capturing pure-function fixtures..."
+
+run_func normalize
+run_func parse_month_references
+run_func calculate_fee
+run_func calculate_junior_fee
+run_func parse_czk_amount
+run_func generate_sync_id
+run_func build_name_variants
+run_func match_members
+run_func infer_transaction_details
+run_func format_date
+
+echo "==> Capturing reconcile fixtures..."
+mkdir -p "$FIXTURES/reconcile"
+eval "$CAPTURE_CMD --func reconcile --all" | while IFS= read -r line; do
+    case_id="$(echo "$line" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d['case'])")"
+    echo "$line" | python3 "$REPO/scripts/scrub_fixtures.py" > "$FIXTURES/reconcile/${case_id}.json"
+done
+
+echo "==> Done. Review with: git diff go/tests/fixtures/"
+echo "==> Audit PII: git ls-files go/tests/fixtures | xargs grep -l '<real name>' should return zero."