startEncounter sets startedAt (clears endedAt on restart). endEncounter sets
endedAt. snapshotOf includes both for undo/redo fidelity. expandUndo split
start/end cases to restore old timestamps.
Display: encounter card shows Started/Ended independently (endedAt shows
even without startedAt). Undo sets null (can't delete via merge), test snap
strips only startedAt/endedAt null for compare.
Reviewed by pi gpt-5.5: stale endedAt on restart fixed, snap null-strip
narrowed. Medium-low risk.