Exercise death-save edge cases in combat replay and scenario tests

Replay (scripts/combat/replay.js):
- Merchant now type=npc via asNpc (was stale isNpc)
- Death-save eligibility keyed on type (character|npc), not removed isNpc
- Rotate outcomes success/fail/nat1/nat20 (was success-only)
- Import stabilizeParticipant, reviveParticipant
- Between-round revive: dead -> reviveParticipant, stable -> heal,
  inactive -> reactivate (heal was no-op on dead, leaving active-dead monsters)
- Fix deathSave return unwrap: it returns {enc, status}; callStep runner
  must extract .enc or state corrupts (round undefined, combat auto-ends)
- describe() fix: death-save arg is outcome, not type

Scenario (src/tests/Combat.scenario.test.js):
- Import stabilizeParticipant, reviveParticipant
- applyDamage accepts options (crit at 0 HP)
- Add stabilizeAction, reviveAction helpers
- addAllCharacters now actually exercises batch add (was no-op)
- New deterministic edge-case test: monster death (dead+inactive),
  NPC nat20, NPC nat1+damage+revive+heal, character stable->damage->crit,
  massive damage

scripts/combat/replay.js

@@ -16,6 +16,7 @@ const {
   startEncounter, nextTurn, togglePause, endEncounter,
   addParticipant, updateParticipant, removeParticipant,
   toggleParticipantActive, applyHpChange, deathSave,
+  stabilizeParticipant, reviveParticipant,
   toggleCondition, reorderParticipants,
 } = shared;
 const { createServerStorage } = require('../../src/storage/server');
@@ -51,7 +52,7 @@ function buildMonsters() {
     { name: 'Goblin2', maxHp: 30, initMod: 2 },
     { name: 'OrcBoss', maxHp: 120, initMod: 1 },
     { name: 'Wolf', maxHp: 40, initMod: 3 },
-    { name: 'Merchant', maxHp: 30, initMod: 0, isNpc: true },
+    { name: 'Merchant', maxHp: 30, initMod: 0, asNpc: true },
   ];
 }
 
@@ -221,7 +222,7 @@ module.exports = async function replay(args) {
           : `${a.target} took ${a.amount} damage`;
       case 'toggleCondition': return `${a.participant} ${a.condition} toggled`;
       case 'updateParticipant': return `${a.participant} edited`;
-      case 'deathSave': return `${a.participant} death save (${a.type})`;
+      case 'deathSave': return `${a.participant} death save (${a.outcome})`;
       case 'toggleParticipantActive':
         return a.revive ? `${a.participant} reactivated` : `${a.participant} toggled`;
       case 'togglePause': return a.to === 'paused' ? 'Combat paused' : 'Combat resumed';
@@ -301,11 +302,14 @@ module.exports = async function replay(args) {
           (e) => updateParticipant(e, actor.id, { notes: `edited r${enc.round}` }, ctx))).enc;
       }
 
-      if (actor.status === 'dying' && !actor.isNpc) {
-        if (VERBOSE) console.log(`    deathSave ${actor.name} +1 success`);
-        enc = (await callStep('deathSave',
-          { participant: actor.name, outcome: 'success' },
-          (e) => deathSave(e, actor.id, 'success', ctx))).enc;
+      if (actor.status === 'dying' && (actor.type === 'character' || actor.type === 'npc')) {
+        const outcomes = ['success', 'fail', 'nat1', 'nat20'];
+        const outcome = outcomes[totalTurns % outcomes.length];
+        if (VERBOSE) console.log(`    deathSave ${actor.name} ${outcome}`);
+        const dsRes = await callStep('deathSave',
+          { participant: actor.name, outcome },
+          async (e) => (await deathSave(e, actor.id, outcome, ctx)).enc);
+        enc = dsRes.enc;
       }
 
       if (totalTurns % 9 === 0) {
@@ -376,19 +380,28 @@ module.exports = async function replay(args) {
       }
       console.log(`--- round ${enc.round} ---`);
       lastRound = enc.round;
-      const dead = enc.participants.filter(p => p.currentHp <= 0 || p.isActive === false);
-      for (const d of dead) {
+      const down = enc.participants.filter(p => p.currentHp <= 0 || p.isActive === false);
+      for (const d of down) {
         if (interrupted) break;
-        if (d.isActive === false) {
-          if (VERBOSE) console.log(`  revive ${d.name}`);
-          enc = (await callStep('toggleParticipantActive',
+        if (d.status === 'dead') {
+          if (VERBOSE) console.log(`  revive dead ${d.name}`);
+          enc = (await callStep('reviveParticipant',
             { participant: d.name, revive: true },
-            (e) => toggleParticipantActive(e, d.id, ctx))).enc;
+            (e) => reviveParticipant(e, d.id, ctx))).enc;
+        }
+        if (d.status === 'stable') {
+          if (VERBOSE) console.log(`  heal stable ${d.name} +${d.maxHp}`);
+          enc = (await callStep('applyHpChange',
+            { target: d.name, changeType: 'heal', amount: d.maxHp, revive: true },
+            (e) => applyHpChange(e, d.id, 'heal', d.maxHp, ctx))).enc;
+        }
+        const latest = (enc.participants || []).find(p => p.id === d.id);
+        if (latest && latest.isActive === false) {
+          if (VERBOSE) console.log(`  reactivate ${latest.name}`);
+          enc = (await callStep('toggleParticipantActive',
+            { participant: latest.name, revive: true },
+            (e) => toggleParticipantActive(e, latest.id, ctx))).enc;
         }
-        if (VERBOSE) console.log(`  heal ${d.name} +${d.maxHp}`);
-        enc = (await callStep('applyHpChange',
-          { target: d.name, changeType: 'heal', amount: d.maxHp, revive: true },
-          (e) => applyHpChange(e, d.id, 'heal', d.maxHp, ctx))).enc;
       }
     }