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

.github/copilot-instructions.md

@@ -12,6 +12,9 @@
 - ✅ **Keep screenshot consent notice in docs** when describing dashboard screenshot feature
 - ✅ **Event-start/event-stop screenshots must preserve metadata** - See SCREENSHOT_MQTT_FIX.md for critical race condition that was fixed
 - ✅ **Screenshot updates must keep `latest.jpg` and `meta.json` in sync** (simclient prefers `latest.jpg`)
+- ✅ **Dashboard payload uses grouped v2 schema** (`message/content/runtime/metadata`, `schema_version="2.0"`)
+- ✅ **Event-triggered screenshots**: `display_manager` arms a `threading.Timer` after start/stop, captures, writes `meta.json` with `send_immediately=true`; simclient fires within ≤1s
+- ✅ **Payload assembly is centralized** in `_build_dashboard_payload()` — do not build dashboard JSON at call sites
 
 ### Key Files & Locations
 - **Display logic**: `src/display_manager.py` (controls presentations/video/web)
@@ -488,31 +491,49 @@ The screenshot capture and transmission system has been implemented with separat
 - **Rotation**: Keeps max N files (default 20), deletes older
 - **Timing**: Production captures when display process is active (unless `SCREENSHOT_ALWAYS=1`); development allows periodic idle captures to keep dashboard fresh
 - **Reliability**: Stale/invalid pending trigger metadata is ignored automatically to avoid lock-up of periodic updates
+- **Event-triggered captures**: `_trigger_event_screenshot(type, delay)` arms a one-shot `threading.Timer` after event start/stop; timer is cancelled and replaced on rapid event switches; default delays: presentation=4s, video=2s, web=5s (env-configurable)
+- **IPC signal file** (`screenshots/meta.json`): written atomically by `display_manager` after each capture; contains `type`, `captured_at`, `file`, `send_immediately`; `send_immediately=true` for event-triggered, `false` for periodic
 
 ### Transmission Strategy (simclient.py)
 - **Source**: Prefers `screenshots/latest.jpg` if present, falls back to newest timestamped file
 - **Topic**: `infoscreen/{client_id}/dashboard`
-- **Format**: JSON with base64-encoded image data
-- **Payload Structure**:
+- **Format**: JSON with base64-encoded image data, grouped v2 schema
+- **Schema version**: `"2.0"` (legacy flat fields removed; all fields grouped)
+- **Payload builder**: `_build_dashboard_payload()` in `simclient.py` — single source of truth
+- **Payload Structure** (v2):
   ```json
   {
-    "timestamp": "ISO datetime",
-    "client_id": "UUID",
-    "status": "alive",
-    "screenshot": {
-      "filename": "latest.jpg",
-      "data": "base64...",
-      "timestamp": "ISO datetime",
-      "size": 12345
+    "message": { "client_id": "UUID", "status": "alive" },
+    "content": {
+      "screenshot": {
+        "filename": "latest.jpg",
+        "data": "base64...",
+        "timestamp": "ISO datetime",
+        "size": 12345
+      }
     },
-    "system_info": {
-      "hostname": "...",
-      "ip": "...",
-      "uptime": 123456.78
+    "runtime": {
+      "system_info": { "hostname": "...", "ip": "...", "uptime": 123456.78 },
+      "process_health": { "event_type": "...", "process_status": "...", ... }
+    },
+    "metadata": {
+      "schema_version": "2.0",
+      "producer": "simclient",
+      "published_at": "ISO datetime",
+      "capture": {
+        "type": "periodic | event_start | event_stop",
+        "captured_at": "ISO datetime",
+        "age_s": 0.9,
+        "triggered": false,
+        "send_immediately": false
+      },
+      "transport": { "topic": "infoscreen/.../dashboard", "qos": 0, "publisher": "simclient" }
     }
   }
   ```
-- **Logging**: Logs publish success/failure with file size for monitoring
+- **Capture types**: `periodic` (interval-based), `event_start` (N seconds after event launch), `event_stop` (1s after process killed)
+- **Triggered send**: `display_manager` sets `send_immediately=true` in `meta.json`; simclient 1-second tick detects and fires within ≤1s
+- **Logging**: `Dashboard published: schema=2.0 type=<type> screenshot=<file> (<bytes>) age=<s>`
 
 ### Scalability Considerations
 - **Client-side resize/compress**: Reduces bandwidth and broker load (recommended for 50+ clients)