validatord: always populate session_id (fallback when Langfuse off)

Surfaced during the 2026-05-02 deploy + reality wave: the persistent
Go stack runs without LANGFUSE_URL/PUBLIC_KEY/SECRET_KEY env, so
shared.langfuseMiddleware operates as a passthrough — never minting
a trace id, never stashing it on the request context. Result:
session_id was empty on every JSONL row, breaking correlation across
the longitudinal log + replay_runs.jsonl + future Langfuse traces.

The fix: validatord falls back to a locally-generated time-ordered
hex id when both the X-Lakehouse-Trace-Id header AND the middleware
context are empty. Same shape Langfuse accepts, so a future deploy
that turns Langfuse on doesn't break correlation — already-emitted
session_ids stay valid as Langfuse trace ids.

Verified post-deploy by driving 9 /v1/iterate sessions through the
persistent stack at :4110:
  - 6 accepted on iter 0 (qwen2.5:latest first-shot 75%)
  - 2 max_iter_exhausted (no_json on prose-y prompts)
  - 1 infra_error (chatd cold-start probe timed out at 5s)

Latest row's session_id: "18abbabdc2306a83-c2306aa9" (was: "")

Probe re-runs (validator_parity, session_log_parity) included as
post-deploy artifacts; both 6/6 + 4/4 with the freshly-restarted
persistent gateway+validatord binaries.

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

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

@@ -1,6 +1,6 @@
 # session_log parity probe — Rust gateway vs Go validatord
 
-**Date:** 2026-05-02T10:38:10Z
+**Date:** 2026-05-02T10:53:00Z
 **Rust helper:** `/home/profit/lakehouse/target/release/parity_session_log`
 **Go helper:** `./bin/parity_session_log_go`
 

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

@@ -1,6 +1,6 @@
 # Validator parity probe — Rust :3100 vs Go :4110
 
-**Date:** 2026-05-02T09:47:49Z
+**Date:** 2026-05-02T10:52:59Z
 **Rust gateway:** `http://127.0.0.1:3100`  ·  **Go gateway:** `http://127.0.0.1:4110`
 
 Identical `POST /v1/validate` request → both runtimes. Match