Avoid false template-render stall timeouts under concurrent generation (#1724)

- Share idle-watchdog progress across in-flight renders and verify expiry against last activity, so large Matter templates like Accessors.cpp are
not killed while sibling renders are still making progress. Also raise the idle window to 2 minutes for safety

- Harden template-render watchdog after review feedback
- Throttle the render-progress broadcast so the per-helper-call path is a single clock read instead of a loop over every in-flight watchdog, which measured 3135ms of pure overhead for 2M helper calls at 300 concurrent renders and 61ms after. Let reset() take a pre-computed timestamp, and recover in expire() when that timestamp is not from performance.now() rather than muting or spuriously triggering the watchdog. Guard the active-watchdog set against a null entry.
- JIRA: ZAPP-1730