parity probes: materializer + extract_json (caught + fixed real bug)

Two new cross-runtime parity probes joining the validator probe from
the gauntlet wave. Pattern: feed identical input through Rust and Go;
diff outputs. Each probe surfaced a different signal.

## Materializer parity probe
scripts/cutover/parity/materializer_parity.sh runs Bun + Go
materializer against an identical synthetic data/_kb/ root, diffs the
resulting evidence/ JSONL byte-equivalent (modulo provenance.recorded_at).

**First run: 0/2 match.** Real finding: Go's Provenance.LineOffset
had `json:"line_offset,omitempty"` which strips the field when value
is 0. Line offset 0 is the FIRST ROW of every source file — a real
semantic value, not absent. Bun side always emits it.

Fix: drop `omitempty` on Provenance.LineOffset. Updated comment
explaining why.

**Re-run: 2/2 match.** On-wire JSON parity holds.

## extract_json parity probe
scripts/cutover/parity/extract_json_parity.sh feeds 12 fixture
strings through both runtimes' extract_json:
  - fenced ```json``` blocks
  - unfenced ``` blocks
  - bare braces with prose around
  - first-balanced-of-many
  - nested objects
  - unicode in string values
  - escaped quotes
  - empty object
  - top-level array (both return first inner object)
  - no JSON
  - depth-balanced but invalid syntax
  - trailing garbage

Substrate gate: cargo test -p gateway extract_json PASS before probe.

**Result: 12/12 match.** Algorithms genuinely equivalent.

## scripts/cutover/parity/extract_json_helper/main.go
Tiny Go binary that reads stdin, calls validator.ExtractJSON, prints
{matched, value} JSON. Counterpart to the Rust parity_extract_json
binary in golangLAKEHOUSE's sibling lakehouse repo (separate commit).

## Pattern crystallized
Every cross-runtime port should land with a parity probe. Three
probes now exist:
  - validator (5/6 wire-format gap captured 2026-05-02)
  - materializer (caught + fixed real bug 2026-05-02)
  - extract_json (12/12 match 2026-05-02)

The instrument is reusable — each new shared HTTP/CLI surface gets
a probe row added.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

reports/cutover/gauntlet_2026-05-02/parity/extract_json_parity.md

@@ -0,0 +1,14 @@
+# extract_json parity probe — Rust vs Go
+
+**Date:** 2026-05-02T09:42:37Z
+**Rust helper:** `/home/profit/lakehouse/target/release/parity_extract_json` (links live `gateway::v1::iterate::extract_json`)
+**Go helper:** `./bin/parity_extract_json_go` (links live `internal/validator.ExtractJSON`)
+
+Identical model-output strings → both runtimes' `extract_json`.
+Match = identical `{matched, value}` JSON output.
+
+**Substrate gate:** `cargo test -p gateway extract_json` PASS before probe.
+
+**Tally:** 12 match · 0 diff (out of 12 fixtures)
+
+_No divergences — extract_json parity holds across all fixtures._

reports/cutover/gauntlet_2026-05-02/parity/materializer_parity.md

@@ -0,0 +1,13 @@
+# Materializer parity probe — Bun vs Go
+
+**Date:** 2026-05-02T09:38:32Z
+**Bun:** `/home/profit/lakehouse/scripts/distillation/build_evidence_index.ts`
+**Go:**  `./bin/materializer`
+
+Identical `data/_kb/` source → both runtimes' materializer.
+Match = JSONL byte-equal after normalizing `provenance.recorded_at`
+(per-run wall clock) + sorted line order (dedup ordering).
+
+**Tally:** 2 match · 0 diff (out of 2 stems)
+
+_No divergences — on-wire JSON parity holds._