Fix turn-usage keying: idempotent per request turn id, not messageId

Live smoke exposed two bugs in the turn usage log:
- onFinish messages carry no ids (withMessageIds fills copies inside
  persistTurn), so fresh turns keyed messageId "" — provenance lost.
- The SDK coalesces continuation turns into ONE response message (a whole
  probe conversation lands in a single assistant message), so per-messageId
  idempotency swallowed every continuation turn's usage.

TurnUsage gains id: string (unique per request, the append idempotency
key); messageId stays as transcript provenance, taken from the id-filled
list. appendTurnUsage dedupes by id; continuation turns with the same
messageId each keep their usage.

src/core/contracts/chat.ts

@@ -93,8 +93,13 @@ export interface CallUsage {
  * agent steps, and each step bills separately. Capture lands at the persist
  * seam (server); cost is INFERRED at read time from core/prices.ts. */
 export interface TurnUsage {
-	/** The assistant message the turn produced — keys this entry to the
-	 * transcript (the block this turn "belongs" to). */
+	/** Unique per REQUEST (turn): one invocation of the handler — the
+	 *  idempotency key for the append-only log. Continuation turns share a
+	 *  response message id (the SDK coalesces them); keying on messageId
+	 *  would swallow their usage. */
+	id: string;
+	/** The assistant message the turn produced — provenance, keys this
+	 *  entry to the transcript (the block this turn "belongs" to). */
 	messageId: string;
 	at: string;
 	/** The OpenRouter model id that ran the turn. */