Add Crit Damage button for dying/stable participants

5e crit-at-0-HP rule: any crit damage while downed = +2 death-save failures.
Shared logic supported via applyHpChange(..., { isCriticalHit: true }) but UI
had no trigger. Normal Damage at 0 only added 1 fail.

handleCritDamage handler calls applyHpChange with isCriticalHit: true.
Button renders in HP block (right-aligned) for character/NPC participants
when dying or stable and combat started. Dead excluded (no-op).

TODO.md

@@ -6,13 +6,6 @@ Backlog of bugs + long-term items. Milestones live in REWORK_PLAN.md.
 
 
 
-### FEAT: critical damage button for dying/stable participants
-- Shared logic already supports `applyHpChange(..., { isCriticalHit:true })`.
-- UI only has normal Damage, which adds 1 failure at 0 HP.
-- Add button near Revive/Stabilize: **Crit Damage** / **Critical Hit**.
-- Use only for dying/stable character/NPC at 0 HP; adds 2 death-save failures.
-- Dead target remains dead; no action.
-
 ### dm list - keep active particpant in view (scroll)
 
 ### combat.js doesnt seem to actually exercise the add characters.  there are no characters in the campaign.   only in the encounter..

src/App.js

@@ -7,7 +7,7 @@ import {
   Play as PlayIcon, Pause as PauseIcon, SkipForward as SkipForwardIcon,
   StopCircle as StopCircleIcon, Users2, Dices, ChevronUp, ChevronDown, ScrollText,
   Maximize2, Minimize2, Moon, Coffee, Clock, ChevronRight, X,
-  Undo2, Redo2
+  Undo2, Redo2, Crosshair
 } from 'lucide-react';
 
 // ----- UI feedback: toast (transient) + info modal (persistent) -----
@@ -1153,6 +1153,16 @@ function ParticipantManager({ encounter, encounterPath, campaignCharacters, camp
     }
   };
 
+  const handleCritDamage = async (participantId) => {
+    if (!db) return;
+    try {
+      // Any damage at 0 HP + isCriticalHit = 2 death-save failures (5e crit-at-0).
+      await combatApplyHpChange(encounter, participantId, 'damage', 1, { isCriticalHit: true }, buildCtx(encounterPath));
+    } catch (err) {
+      showToast(err.message);
+    }
+  };
+
   const toggleCondition = async (participantId, conditionId) => {
     if (!db) return;
     try {
@@ -1464,6 +1474,11 @@ function ParticipantManager({ encounter, encounterPath, campaignCharacters, camp
                         />
                       </span>
                       <span>HP: {p.currentHp}/{p.maxHp}</span>
+                      {hasDeathSaves && (participantStatus === 'dying' || participantStatus === 'stable') && encounter.isStarted && (
+                        <button onClick={() => handleCritDamage(p.id)} className="ml-auto px-2 py-0.5 text-xs rounded bg-red-900 hover:bg-red-800 text-red-100 border border-red-700" title="Critical hit at 0 HP: +2 death-save failures">
+                          <Crosshair size={12} className="inline mr-1" />Crit
+                        </button>
+                      )}
                     </div>
                     
                     {participantStatus === 'dead' && (