gateway: pub extract_json + parity_extract_json bin (cross-runtime probe)

Supports the 2026-05-02 cross-runtime parity probe at
golangLAKEHOUSE/scripts/cutover/parity/extract_json_parity.sh which
feeds identical model-output strings through both runtimes' extract_json
and diffs results.

## Changes
- crates/gateway/src/v1/iterate.rs: extract_json gains `pub` + a
  comment pointing at the Go counterpart and the parity probe path
- crates/gateway/src/lib.rs: NEW thin lib facade re-exporting the
  modules so sub-binaries can reuse them. main.rs is unchanged
  (still uses local mod declarations)
- crates/gateway/src/bin/parity_extract_json.rs: NEW ~30-LOC binary
  that reads stdin, calls extract_json, prints {matched, value} JSON

## Probe result (logged in golangLAKEHOUSE)
12/12 match across fenced blocks, nested objects, unicode, escaped
quotes, top-level array, malformed JSON. Both runtimes' algorithms
are genuinely equivalent.

Substrate gate the probe enforces: `cargo test -p gateway extract_json`
PASS before any parity comparison runs. So a future divergence in
the live extract_json fires either as a Rust test failure (live
behavior changed) or a probe diff (Go behavior changed) — never
silently.

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

crates/gateway/src/v1/iterate.rs

@@ -234,7 +234,13 @@ async fn call_validate(client: &reqwest::Client, gateway: &str, body: &serde_jso
 /// Extract the first JSON object from a model's output. Handles
 /// fenced code blocks (```json ... ```), bare braces, and stray
 /// prose around the JSON. Returns None on no extractable object.
-fn extract_json(raw: &str) -> Option<serde_json::Value> {
+///
+/// Made `pub` 2026-05-02 to support the cross-runtime parity probe
+/// at `golangLAKEHOUSE/scripts/cutover/parity/extract_json_parity.sh`.
+/// The Go counterpart lives at `internal/validator/iterate.go::ExtractJSON`;
+/// when either runtime's algorithm changes the parity probe surfaces
+/// the divergence.
+pub fn extract_json(raw: &str) -> Option<serde_json::Value> {
     // Try fenced first.
     let candidates: Vec<String> = {
         let mut out = vec![];