Track encounter start/end timestamps; display on card

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.

shared/tests/turn.undo.test.js

@@ -29,7 +29,13 @@ function enc(ps) {
   return { name:'t', participants:ps, isStarted:false, isPaused:false,
     round:0, currentTurnParticipantId:null, turnOrderIds:[] };
 }
-const snap = (e) => JSON.parse(JSON.stringify(e));
+const snap = (e) => {
+  const o = JSON.parse(JSON.stringify(e));
+  // normalize nullable timestamp fields: null = absent (undo clears via null)
+  if (o.startedAt === null) delete o.startedAt;
+  if (o.endedAt === null) delete o.endedAt;
+  return o;
+};
 
 describe('undo + redo roundtrip', () => {
   test('startEncounter', async () => {

shared/turn.js

@@ -102,6 +102,8 @@ function snapshotOf(enc) {
     currentTurnParticipantId: enc.currentTurnParticipantId ?? null,
     turnOrderIds: [...(enc.turnOrderIds || [])],
     activeIds: (enc.participants || []).filter(p => p.isActive).map(p => p.id),
+    startedAt: enc.startedAt ?? null,
+    endedAt: enc.endedAt ?? null,
   };
 }
 
@@ -305,12 +307,18 @@ function expandUndo(entry, currentEnc) {
         redo: { currentTurnParticipantId: snap.currentTurnParticipantId, round: snap.round, turnOrderIds: snap.turnOrderIds || [] },
       };
     }
-    case 'start_encounter':
+    case 'start_encounter': {
+      return {
+        encounterPath: encPath,
+        updates: { ...u, startedAt: u.startedAt ?? null },
+        redo: { isStarted: true, isPaused: false, currentTurnParticipantId: snap.currentTurnParticipantId, round: snap.round, turnOrderIds: snap.turnOrderIds || [], startedAt: snap.startedAt ?? Date.now() },
+      };
+    }
     case 'end_encounter': {
       return {
         encounterPath: encPath,
-        updates: u,
+        updates: { ...u, endedAt: u.endedAt ?? null },
-        redo: { isStarted: entry.type === 'start_encounter', isPaused: false, currentTurnParticipantId: snap.currentTurnParticipantId, round: snap.round, turnOrderIds: snap.turnOrderIds || [] },
+        redo: { isStarted: false, isPaused: false, currentTurnParticipantId: snap.currentTurnParticipantId, round: snap.round, turnOrderIds: snap.turnOrderIds || [], endedAt: snap.endedAt ?? Date.now() },
       };
     }
     default:
@@ -391,6 +399,8 @@ async function startEncounter(encounter, ctx) {
     participants: sortedParticipants,
     currentTurnParticipantId: firstActive.id,
     turnOrderIds: sortedParticipants.map(p => p.id),
+    startedAt: Date.now(),
+    endedAt: null,
   };
   const log = {
     type: 'start_encounter',
@@ -403,6 +413,8 @@ async function startEncounter(encounter, ctx) {
       round: encounter.round ?? 0,
       currentTurnParticipantId: encounter.currentTurnParticipantId ?? null,
       turnOrderIds: [...(encounter.turnOrderIds || [])],
+      startedAt: encounter.startedAt ?? null,
+      endedAt: encounter.endedAt ?? null,
     },
   };
   return commit(encounter, patch, log, ctx);
@@ -1003,12 +1015,12 @@ async function reorderParticipants(encounter, draggedId, targetId, ctx) {
 }
 
 async function endEncounter(encounter, ctx) {
-  const patch = { isStarted: false, isPaused: false, currentTurnParticipantId: null, round: 0, turnOrderIds: [] };
+  const patch = { isStarted: false, isPaused: false, currentTurnParticipantId: null, round: 0, turnOrderIds: [], endedAt: Date.now() };
   const log = {
     type: 'end_encounter',
     message: `Combat ended: "${encounter.name}"`,
     delta: {},
-    undo: { isStarted: encounter.isStarted ?? false, isPaused: encounter.isPaused ?? false, round: encounter.round ?? 0, currentTurnParticipantId: encounter.currentTurnParticipantId ?? null, turnOrderIds: [...(encounter.turnOrderIds || [])] },
+    undo: { isStarted: encounter.isStarted ?? false, isPaused: encounter.isPaused ?? false, round: encounter.round ?? 0, currentTurnParticipantId: encounter.currentTurnParticipantId ?? null, turnOrderIds: [...(encounter.turnOrderIds || [])], endedAt: encounter.endedAt ?? null },
   };
   return commit(encounter, patch, log, ctx);
 }

src/App.js

@@ -2258,6 +2258,11 @@ function EncounterManager({ campaignId, initialActiveEncounterId, campaignCharac
                     <p className="text-xs text-stone-300">
                       {encounter.createdAt && `${new Date(encounter.createdAt).toLocaleDateString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })} · `}Participants: {encounter.participants?.length || 0}
                     </p>
+                    {(encounter.startedAt || encounter.endedAt) && (
+                      <p className="text-xs text-green-400/80 mt-0.5">
+                        {encounter.startedAt && `⚔ Started: ${new Date(encounter.startedAt).toLocaleString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })}`}{encounter.startedAt && encounter.endedAt && ' · '}{encounter.endedAt && `Ended: ${new Date(encounter.endedAt).toLocaleString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false })}`}
+                      </p>
+                    )}
                     {isLive && (
                       <span className="text-xs text-green-400 font-semibold block mt-1">
                         LIVE ON PLAYER DISPLAY