FN-5920: prefer mission detail event count for activity tab

Use mission detail event counts as the authoritative pre-load source for mission activity.

- add mission hierarchy eventCount support in core and dashboard mission types
- return mission eventCount from mission detail APIs and document the field
- prefer the detail eventCount over stale list summaries before activity events load
- add coverage for store, API, and MissionManager pre-load count behavior

Files changed:
 docs/missions.md                                   |  2 +-
 packages/core/src/__tests__/mission-store.test.ts  | 18 +++++++++
 packages/core/src/mission-store.ts                 |  6 +++
 packages/core/src/mission-types.ts                 |  2 +
 packages/dashboard/app/api/legacy.ts               |  2 +
 packages/dashboard/app/components/MissionManager.tsx    | 12 +++++-
 packages/dashboard/app/components/__tests__/MissionManager.test.tsx   | 46 +++++++++++++++++++++-
 packages/dashboard/app/components/mission-types.ts |  1 +
 packages/dashboard/src/__tests__/mission-e2e.test.ts    |  2 +
 9 files changed, 86 insertions(+), 5 deletions(-)

Fusion-Task-Id: FN-5920

Fusion-Task-Lineage: a0269499-d34a-495d-89fc-0534bb62a94e

packages/dashboard/app/api/legacy.ts

@@ -6980,6 +6980,8 @@ export interface SliceWithFeatures extends Slice {
 
 /** Full mission hierarchy */
 export interface MissionWithHierarchy extends Mission {
+  /** Unfiltered total of all mission lifecycle events, matching MissionSummary.eventCount and getMissionEvents total with no eventType filter */
+  eventCount?: number;
   milestones: MilestoneWithSlices[];
 }