gateway: /v1/validate endpoint — Phase 43 v3 part 2

Closes the Phase 43 PRD's "any caller can validate" surface. The
validator crate (FillValidator + EmailValidator + PlaybookValidator
+ WorkerLookup) is now reachable over HTTP at /v1/validate.

Request/response:
  POST /v1/validate
    {"kind":"fill"|"email"|"playbook", "artifact":{...}, "context":{...}?}
  → 200 + Report on success
  → 422 + ValidationError on validation failure
  → 400 on bad kind

Boot-time wiring (main.rs):
- Load workers_500k.parquet into a shared Arc<dyn WorkerLookup>
- Path overridable via LH_WORKERS_PARQUET env
- Missing file: warn + fall back to empty InMemoryWorkerLookup so the
  endpoint stays live (validators just fail Consistency on every
  worker-existence check, which is the correct behavior when the
  roster isn't configured)
- Boot log line: "workers parquet loaded from <path>" or
  "workers parquet at <path> not found"
- Live boot timing: 500K rows loaded in ~1.4s

V1State gains `validate_workers: Arc<dyn validator::WorkerLookup>`.
The `_context` JSON key is auto-injected from `request.context` so
callers can either embed `_context` directly in `artifact` or split
it cleanly via the `context` field.

Verified live (gateway + 500K worker snapshot):
  POST {kind:"fill", phantom W-FAKE-99999}    → 422 Consistency
                                                 ("does not exist in
                                                  worker roster")
  POST {kind:"fill", real W-1, "Anyone"}      → 200 OK + Warning
                                                 ("differs from
                                                  roster name 'Donald
                                                  Green'")
  POST {kind:"email", body has 123-45-6789}   → 422 Policy ("SSN-
                                                shaped sequence")
  POST {kind:"nonsense"}                       → 400 Bad Request

The "0→85% with iteration" thesis can now run end-to-end on real
staffing data: an executor emits a fill_proposal, posts to
/v1/validate, gets a structured ValidationError on phantom IDs or
inactive workers, observer-corrects, retries. Closure of that loop
in a scrum harness is the next commit (separate scope).

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

crates/gateway/src/v1/mod.rs

@@ -18,6 +18,7 @@ pub mod gemini;
 pub mod claude;
 pub mod kimi;
 pub mod opencode;
+pub mod validate;
 pub mod langfuse_trace;
 pub mod mode;
 pub mod respond;
@@ -68,6 +69,15 @@ pub struct V1State {
     /// `OPENCODE_API_KEY` env or `/etc/lakehouse/opencode.env`. None =
     /// provider="opencode" calls 503.
     pub opencode_key: Option<String>,
+    /// Shared WorkerLookup loaded once at startup from
+    /// workers_500k.parquet (path: LH_WORKERS_PARQUET env, default
+    /// data/datasets/workers_500k.parquet). Used by /v1/validate to
+    /// run FillValidator/EmailValidator with worker-existence checks.
+    /// Falls back to an empty InMemoryWorkerLookup if the file is
+    /// missing — validators still run schema/PII checks but every
+    /// worker-existence check fails (Consistency error), which is
+    /// the correct behavior when the roster isn't configured.
+    pub validate_workers: std::sync::Arc<dyn validator::WorkerLookup>,
     /// Phase 40 early deliverable — Langfuse client. None = tracing
     /// disabled (keys missing or container unreachable). Traces are
     /// fire-and-forget: never block the response path.
@@ -107,6 +117,7 @@ pub fn router(state: V1State) -> Router {
         .route("/mode", post(mode::route))
         .route("/mode/list", get(mode::list))
         .route("/mode/execute", post(mode::execute))
+        .route("/validate", post(validate::validate))
         .with_state(state)
 }