gateway: IterateResponse echoes trace_id + enable session_log_path

Closes the 2026-05-02 cross-runtime parity gap: Go's
validator.IterateResponse carried trace_id back to callers; Rust's
didn't. A caller pivoting from response → Langfuse → session log
worked on Go but failed on Rust because the join key wasn't visible
in the response body.

## Changes

crates/gateway/src/v1/iterate.rs:
  - IterateResponse + IterateFailure gain `trace_id: Option<String>`
    (skip-serializing-if-none preserves backward-compat for any
    consumer parsing the response without the field)
  - Both return sites populated with the resolved trace_id

lakehouse.toml:
  - [gateway].session_log_path set to /tmp/lakehouse-validator/sessions.jsonl
    — same path Go validatord writes to. The two daemons now co-write
    one unified longitudinal log; rows tag daemon="gateway" vs
    daemon="validatord" so producers stay distinguishable in DuckDB
    queries. Append-write is atomic at the row sizes both runtimes
    produce, so concurrent writes from both daemons are safe.

## Verification

Post-restart of lakehouse.service:
  POST /v1/iterate with X-Lakehouse-Trace-Id: rust-fix1-test
    → response.trace_id = "rust-fix1-test" ✓ (was: field absent)
    → sessions.jsonl latest row daemon=gateway, session_id=rust-fix1-test ✓ (was: no row)

Cross-runtime drive — same prompt to Rust :3100 and Go :4110:
  Rust:  trace_id=unified-rust-001, daemon=gateway, accepted
  Go:    trace_id=unified-go-001,   daemon=validatord, accepted
  Same file, distinct daemons, one query covers both:
    SELECT daemon, COUNT(*) FROM read_json_auto('sessions.jsonl', format='nd') GROUP BY daemon
    → gateway: 2, validatord: 19

All 4 parity probes still 6/6 + 12/12 + 4/4 + 2/2 against live
:3100 + :4110 stacks. Cargo test 4/4 PASS for v1::iterate module.

## Architecture invariant

The "unified longitudinal log" thesis is now demonstrated. Operators
running both runtimes in production point both daemons at the same
session_log_path and DuckDB queries naturally span both producers.

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

crates/gateway/src/v1/iterate.rs

@@ -87,6 +87,14 @@ pub struct IterateResponse {
     pub validation: serde_json::Value,
     pub iterations: u32,
     pub history: Vec<IterateAttempt>,
+    /// Echoes the resolved trace id (caller-forwarded header, body
+    /// field, langfuse-middleware mint, or local fallback). Operators
+    /// pivot from this id straight into Langfuse + the
+    /// coordinator_sessions.jsonl join key. Cross-runtime parity with
+    /// Go's `validator.IterateResponse` (commit 6847bbc in
+    /// golangLAKEHOUSE).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub trace_id: Option<String>,
 }
 
 #[derive(Serialize)]
@@ -94,6 +102,8 @@ pub struct IterateFailure {
     pub error: String,
     pub iterations: u32,
     pub history: Vec<IterateAttempt>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub trace_id: Option<String>,
 }
 
 pub async fn iterate(
@@ -217,6 +227,7 @@ pub async fn iterate(
                     validation: report,
                     iterations: iteration + 1,
                     history,
+                    trace_id: Some(trace_id.clone()),
                 })).into_response();
             }
             Err(err) => {
@@ -259,6 +270,7 @@ pub async fn iterate(
         error: format!("max iterations reached ({max_iter}) without passing validation"),
         iterations: max_iter,
         history,
+        trace_id: Some(trace_id.clone()),
     })).into_response()
 }
 

lakehouse.toml

@@ -5,9 +5,13 @@ host = "0.0.0.0"
 port = 3100
 # Coordinator session JSONL — one row per /v1/iterate session for
 # offline DuckDB analysis. Cross-runtime parity with the Go-side
-# [validatord].session_log_path. Empty = disabled. Production:
+# [validatord].session_log_path. Set to the SAME path Go validatord
-#   session_log_path = "/var/lib/lakehouse/gateway/sessions.jsonl"
+# writes to so DuckDB queries see one unified longitudinal stream
-session_log_path = ""
+# across both runtimes (rows are tagged daemon="gateway" vs
+# daemon="validatord" so producers stay distinguishable). Append-write
+# is atomic at the row sizes both runtimes produce — both daemons
+# co-writing is safe.
+session_log_path = "/tmp/lakehouse-validator/sessions.jsonl"
 
 [storage]
 root = "./data"