feat(calibrate-serve): POST /room/train + fix AnchorLabel JSON to snake_case

- POST /api/v1/room/train: { room_id, baseline_id, anchors[] } → trains a
  SpecialistBank and persists it as <output_dir>/<room_id>.json (path-sanitized),
  readable via /room/state?bank=<room_id>. Completes the HTTP train→infer loop.
- Fix data-contract bug: AnchorLabel serialized as PascalCase variant names
  (serde default) while as_str() + the integration doc used snake_case. Added
  #[serde(rename_all = "snake_case")] so the JSON wire format matches the
  documented contract (empty/stand_still/…). Locked with a roundtrip test.

Validated live (ESP32-S3): POST train (4 anchors → 6 specialists, persisted) →
GET /room/state returns RoomState with the trained presence/restlessness; the
synthetic-vs-real scale mismatch correctly triggers the anomaly veto. 36
calibration tests pass.

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/crates/wifi-densepose-calibration/src/anchor.rs

@@ -20,7 +20,11 @@ pub enum Posture {
 }
 
 /// The fixed guided-anchor sequence (ADR-151 §2.2).
+///
+/// Serializes as snake_case (`empty`, `stand_still`, …) to match
+/// [`AnchorLabel::as_str`] and the documented JSON contract.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
 pub enum AnchorLabel {
     /// Empty room reference (reuses the ADR-135 baseline).
     Empty,
@@ -284,6 +288,17 @@ mod tests {
         assert_eq!(AnchorLabel::from_str("nope"), None);
     }
 
+    #[test]
+    fn label_serde_is_snake_case_matching_as_str() {
+        // The JSON wire format must equal as_str() (the documented contract).
+        for l in AnchorLabel::SEQUENCE {
+            let json = serde_json::to_string(&l).unwrap();
+            assert_eq!(json, format!("\"{}\"", l.as_str()));
+            let back: AnchorLabel = serde_json::from_str(&json).unwrap();
+            assert_eq!(back, l);
+        }
+    }
+
     #[test]
     fn sequence_order_and_next() {
         let mut s = EnrollmentSession::new("living-room", "base-1", 0);