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

MQTT_PAYLOAD_MIGRATION_CHECKLIST.md

@@ -4,43 +4,43 @@ Use this checklist to migrate from legacy flat dashboard payload to grouped v2 p
 
 ## A. Client Implementation
 
-- [ ] Create branch for migration work.
-- [ ] Capture one baseline message from MQTT (legacy format).
-- [ ] Implement one canonical payload builder function.
-- [ ] Emit grouped blocks in this order: `message`, `content`, `runtime`, `metadata`.
-- [ ] Add `metadata.schema_version = "2.0"`.
-- [ ] Add `metadata.producer = "simclient"`.
-- [ ] Add `metadata.published_at` in UTC ISO format.
-- [ ] Map capture type to `metadata.capture.type` (`periodic`, `event_start`, `event_stop`).
-- [ ] Map screenshot freshness to `metadata.capture.age_s`.
-- [ ] Keep screenshot object unchanged in semantics (`filename`, `data`, `timestamp`, `size`).
-- [ ] Keep trigger behavior unchanged (periodic and triggered sends still work).
-- [ ] Add publish log fields: schema version, capture type, age.
-- [ ] Validate all 3 paths end-to-end:
-  - [ ] periodic
-  - [ ] event_start
-  - [ ] event_stop
+- [x] Create branch for migration work.
+- [x] Capture one baseline message from MQTT (legacy format).
+- [x] Implement one canonical payload builder function.
+- [x] Emit grouped blocks in this order: `message`, `content`, `runtime`, `metadata`.
+- [x] Add `metadata.schema_version = "2.0"`.
+- [x] Add `metadata.producer = "simclient"`.
+- [x] Add `metadata.published_at` in UTC ISO format.
+- [x] Map capture type to `metadata.capture.type` (`periodic`, `event_start`, `event_stop`).
+- [x] Map screenshot freshness to `metadata.capture.age_s`.
+- [x] Keep screenshot object unchanged in semantics (`filename`, `data`, `timestamp`, `size`).
+- [x] Keep trigger behavior unchanged (periodic and triggered sends still work).
+- [x] Add publish log fields: schema version, capture type, age.
+- [x] Validate all 3 paths end-to-end:
+  - [x] periodic
+  - [x] event_start
+  - [x] event_stop
 
 ## B. Server Migration
 
-- [ ] Add grouped v2 parser (`message/content/runtime/metadata`).
-- [ ] Add temporary legacy fallback parser.
-- [ ] Normalize both parsers into one internal server model.
-- [ ] Mark required fields:
-  - [ ] `message.client_id`
-  - [ ] `message.status`
-  - [ ] `metadata.schema_version`
-  - [ ] `metadata.capture.type`
-- [ ] Keep optional fields tolerated (`runtime.process_health`, `content.screenshot`).
-- [ ] Update dashboard consumers to use normalized model (not raw legacy keys).
-- [ ] Add migration counters:
-  - [ ] v2 parse success
-  - [ ] legacy fallback usage
-  - [ ] parse failures
-- [ ] Test compatibility matrix:
-  - [ ] new client -> new server
-  - [ ] legacy client -> new server
-- [ ] Run short soak in dev.
+- [x] Add grouped v2 parser (`message/content/runtime/metadata`).
+- [x] Add temporary legacy fallback parser.
+- [x] Normalize both parsers into one internal server model.
+- [x] Mark required fields:
+  - [x] `message.client_id`
+  - [x] `message.status`
+  - [x] `metadata.schema_version`
+  - [x] `metadata.capture.type`
+- [x] Keep optional fields tolerated (`runtime.process_health`, `content.screenshot`).
+- [x] Update dashboard consumers to use normalized model (not raw legacy keys).
+- [x] Add migration counters:
+  - [x] v2 parse success
+  - [x] legacy fallback usage
+  - [x] parse failures
+- [x] Test compatibility matrix:
+  - [x] new client -> new server
+  - [x] legacy client -> new server
+- [x] Run short soak in dev.
 
 ## C. Cutover and Cleanup