Idempotency under a retry storm

Click "Retry x5 (same key)" to fire five rapid RPCs with the same idempotencyKey. Only the first runs the handler; the other four return the cached result. One intent, one effect.

Both buttons disable while a call is in flight, so a literal double-click is stopped by the page before it reaches the server - that is UI lockout, not idempotency. The burst above is the honest demonstration: five real overlapping RPCs the client never suppressed, deduped by their shared key on the server.