feat(mqtt): finalize dashboard screenshot payload v2 and trigger flow

- switch dashboard payload to grouped schema v2.0 in simclient
- support immediate event-triggered screenshot sends via meta.json signaling
- update README and copilot instructions to document v2 payload and trigger behavior
- update migration checklist to reflect completed client/server rollout

README.md

@@ -394,7 +394,7 @@ The MQTT client ([src/simclient.py](src/simclient.py)) downloads presentation fi
 #### Client → Server
 - `infoscreen/discovery` - Initial client announcement
 - `infoscreen/{client_id}/heartbeat` - Regular status updates
-- `infoscreen/{client_id}/dashboard` - Screenshot images (base64)
+- `infoscreen/{client_id}/dashboard` - Dashboard payload v2 (grouped schema: message/content/runtime/metadata, includes screenshot base64, capture type, schema version)
 - `infoscreen/{client_id}/health` - Process health state (`event_id`, process, pid, status)
 - `infoscreen/{client_id}/logs/error` - Forwarded client error logs
 - `infoscreen/{client_id}/logs/warn` - Forwarded client warning logs
@@ -587,7 +587,8 @@ stat src/screenshots/latest.jpg
 **Verify simclient is reading screenshots:**
 ```bash
 tail -f logs/simclient.log | grep -i screenshot
-# Should show: "Dashboard heartbeat sent with screenshot: latest.jpg"
+# Should show: "Dashboard published: schema=2.0 type=periodic screenshot=latest.jpg"
+# For event transitions: "Dashboard published: schema=2.0 type=event_start ..."
 ```
 
 ## 📚 Documentation
@@ -771,3 +772,8 @@ For issues or questions:
 - Stale/invalid pending trigger metadata now self-heals instead of blocking periodic updates.
 - Display environment fallbacks (`DISPLAY=:0`, `XAUTHORITY`) improved for non-interactive starts.
 - Development mode allows periodic idle captures to keep dashboard previews fresh when no event is active.
+- Event-triggered screenshots added: `display_manager` captures a screenshot shortly after every event start and stop and signals `simclient` via `meta.json` (`send_immediately=true`). Capture delays are content-type aware (presentation: 4s, video: 2s, web: 5s, configurable via `.env`).
+- `simclient` screenshot service thread now runs on a 1-second tick instead of a blocking sleep, so triggered sends fire within ≤1s of the `meta.json` signal.
+- Dashboard payload migrated to grouped v2 schema (`message`, `content`, `runtime`, `metadata`). Legacy flat fields removed. `metadata.schema_version` is `"2.0"`. Payload assembly centralized in `_build_dashboard_payload()`.
+- Tunable trigger delays added: `SCREENSHOT_TRIGGER_DELAY_PRESENTATION`, `SCREENSHOT_TRIGGER_DELAY_VIDEO`, `SCREENSHOT_TRIGGER_DELAY_WEB`.
+- Rapid event switches handled safely: pending trigger timer is cancelled and replaced when a new event starts before the delay expires.