feat: support extra health state mechanics

docs/TALENT_MECHANIC_RULES.md

@@ -26,7 +26,14 @@ Use these event boundaries:
 - **Healing:** Count each healing event. A heal-over-time effect counts each
   tick as a separate event.
 - **Extra Health gained:** Count every Extra Health grant, including a grant
-  received while the Agent already has Extra Health.
+  received while the Agent already has Extra Health. Use
+  `extra_health_granted` for the grant action.
+- **Extra Health state changed:** `extra_health_changed` is Agent-scoped. Emit
+  it when the Agent receives a grant, when the remaining amount changes, and
+  when it is depleted to zero. Every event requires the boolean
+  `values.hasExtraHealth`. It may include the non-negative
+  `values.currentExtraHealth`, which must agree with that status. This state
+  event is distinct from the `extra_health_granted` grant action.
 - **Reloading:** Trigger on reload completion when the weapon had at least one
   round to load. A canceled reload does not count. A weapon that loads one
   round at a time produces one completed-reload event for each round.