docs: close 2 cross-runtime parity gaps + document unified log

Companion to lakehouse 98b6647. Architecture comparison decisions
tracker now captures:

  - Go validatord direct header read (fixes 6847bbc): closes the
    case where Langfuse-off middleware passthrough silently dropped
    forwarded X-Lakehouse-Trace-Id
  - Rust IterateResponse trace_id echo (fixes 98b6647): closes the
    asymmetry where Go's response carried the join key and Rust's
    didn't
  - Unified longitudinal log demonstrated end-to-end: both daemons
    co-writing /tmp/lakehouse-validator/sessions.jsonl, distinct
    daemon tags, one DuckDB query covers both

24/24 parity assertions (validator 6/6, extract_json 12/12,
session_log 4/4, materializer 2/2) hold against live :3100 + :4110.
Both runtimes deployed with today's full stack.

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

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

@@ -1,6 +1,6 @@
 # extract_json parity probe — Rust vs Go
 
-**Date:** 2026-05-02T09:42:37Z
+**Date:** 2026-05-02T11:24:21Z
 **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`)
 

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

@@ -1,6 +1,6 @@
 # Materializer parity probe — Bun vs Go
 
-**Date:** 2026-05-02T09:38:32Z
+**Date:** 2026-05-02T11:24:21Z
 **Bun:** `/home/profit/lakehouse/scripts/distillation/build_evidence_index.ts`
 **Go:**  `./bin/materializer`
 

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:** 2026-05-02T10:53:00Z
+**Date:** 2026-05-02T11:24:21Z
 **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:** 2026-05-02T10:52:59Z
+**Date:** 2026-05-02T11:24:15Z
 **Rust gateway:** `http://127.0.0.1:3100`  ·  **Go gateway:** `http://127.0.0.1:4110`
 
 Identical `POST /v1/validate` request → both runtimes. Match