fix: end combat failing when character sync enabled

Character writeback in endEncounter wrote to a bare campaigns/{id} path,
which the firebase adapter passes straight to the SDK — the update hit a
nonexistent top-level doc, threw, and aborted the whole end-combat action
(round never reset, combat stayed running).

- endEncounter now uses ctx.campaignPath (full artifacts/... path from
  App.js), falling back to the bare path for server/test adapters
- writeback failures no longer block ending combat
- regression tests: campaignPath routing, writeback-failure resilience,
  and participants never cleared on end

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shared/turn.js

@@ -1129,11 +1129,15 @@ async function endEncounter(encounter, ctx) {
   };
 
   // Character writeback: sync maxHp/ac/currentHp to campaign players if enabled.
+  // ctx.campaignPath: adapter-specific doc path (firebase needs the full
+  // artifacts/{APP_ID}/public/data prefix); bare campaigns/{id} fallback for
+  // server/test adapters. Writeback failure must not block ending combat.
   const wbCampaignId = encounter.campaignId || ctx.campaignId;
   if ((wbCampaignId || ctx.campaign) && ctx.storage) {
+    const campaignPath = ctx.campaignPath || `campaigns/${wbCampaignId}`;
     let campaign;
     try {
-      campaign = ctx.campaign || await ctx.storage.getDoc(`campaigns/${wbCampaignId}`);
+      campaign = ctx.campaign || await ctx.storage.getDoc(campaignPath);
     } catch { campaign = null; }
     if (campaign && campaign.syncCharacters && Array.isArray(campaign.players)) {
       const charParticipants = (encounter.participants || []).filter(p => p.originalCharacterId);
@@ -1153,8 +1157,12 @@ async function endEncounter(encounter, ctx) {
         return newPlayer;
       });
       if (changed) {
-        await ctx.storage.updateDoc(`campaigns/${wbCampaignId}`, { players: updatedPlayers });
-        log.undo.characterWriteback = oldValues;
+        try {
+          await ctx.storage.updateDoc(campaignPath, { players: updatedPlayers });
+          log.undo.characterWriteback = oldValues;
+        } catch (err) {
+          console.error(`endEncounter: character writeback failed (${campaignPath}):`, err);
+        }
       }
     }
   }