gateway+validator: /v1/health reports honest worker count for production

Adds `fn len() -> usize` (default 0) to the WorkerLookup trait. The
InMemoryWorkerLookup overrides with HashMap size; ParquetWorkerLookup
constructs an InMemoryWorkerLookup so it inherits the count.

/v1/health now reports `workers_count` (exact integer) alongside
`workers_loaded` (derived bool: count > 0). The previous placeholder
true was a known caveat in the prior commit's body — this closes it.

Production switchover use case: J swaps workers_500k.parquet → real
Chicago contractor data, restarts the gateway, and verifies the
swap with one curl:

  curl http://localhost:3100/v1/health | jq .workers_count

Expected: matches the row count of the new file. Mismatch (or 0)
means the file is missing / unreadable / had a schema mismatch and
the gateway fell back to the empty InMemoryWorkerLookup. Operator
catches the drift before traffic reaches the validators.

Verified live (current synthetic data):
  workers_count: 500000   (matches workers_500k.parquet row count)
  workers_loaded: true

When the Chicago data lands, the same curl is the single source of
truth that the new dataset is hot. Removes the
restart-and-pray failure mode.

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

crates/validator/src/lib.rs

@@ -130,6 +130,16 @@ pub struct WorkerRecord {
 /// hold an in-memory snapshot, not perform per-call I/O.
 pub trait WorkerLookup: Send + Sync {
     fn find(&self, candidate_id: &str) -> Option<WorkerRecord>;
+    /// Number of workers in the snapshot. Default 0 for impls that
+    /// genuinely don't know (e.g. a future SQL-backed lookup that
+    /// counts on demand). InMemoryWorkerLookup overrides with the
+    /// HashMap size; ParquetWorkerLookup constructs an
+    /// InMemoryWorkerLookup so it inherits the override. Used by
+    /// /v1/health to report data-load status during production
+    /// switchover (the Chicago dataset replaces synthetic test data;
+    /// the health endpoint is how operators verify the new file
+    /// loaded correctly without restart-and-pray).
+    fn len(&self) -> usize { 0 }
 }
 
 /// HashMap-backed lookup. Used by validator unit tests + as a
@@ -165,4 +175,7 @@ impl WorkerLookup for InMemoryWorkerLookup {
     fn find(&self, candidate_id: &str) -> Option<WorkerRecord> {
         self.rows.get(candidate_id).cloned()
     }
+    fn len(&self) -> usize {
+        self.rows.len()
+    }
 }