V2 Phase 2: spell + resource enforcement (sheet → combat → live)

The Slice-0 vertical that proves the enforcement architecture end-to-end.

- kernel: castSpell (slot consumption + concentration set/replace, upcasting,
  warlock pact slots), spendResource/regainResource, rollDeathSave — pure
  functions returning Partial<Character> patches. 13 unit tests.
- applyRest now ends concentration on any rest (+ test).
- character sheet SpellcastingSection: per-spell Cast button, concentration
  banner with Drop, concentration markers; ResourcesSection −/+ routed through
  the kernel.
- player MyCharacterPanel: castable spell list + concentration banner, so a
  seated player casts and it broadcasts via the existing playerPatch flow.
- live sync: partialCharacterDiffSchema carries concentration; GM applies it;
  playerCharacterSchema mirrors slots/concentration/resources (PC-only) and the
  player-facing AC now respects equippedArmor. Wire round-trip test added.

Verified live: casting decrements the right slot, switching concentration logs
"Concentration on X ends", banner + Drop render. Build + 258 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/features/characters/sheet/ResourcesSection.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { newId } from '@/lib/ids';
 import { getSystem, applyRest } from '@/lib/rules';
+import { spendResource, regainResource } from '@/lib/mechanics';
 import type { CharacterResource } from '@/lib/schemas';
 import { Button } from '@/components/ui/Button';
 import { Input, Select } from '@/components/ui/Input';
@@ -61,12 +62,12 @@ export function ResourcesSection({ c, update }: SectionProps) {
           {c.resources.map((r) => (
             <li key={r.id} className="flex items-center gap-2 rounded-md border border-line bg-panel px-3 py-2">
               <Input className="h-8 flex-1" value={r.name} onChange={(e) => patch(r.id, { name: e.target.value })} aria-label="Resource name" />
-              <Button size="icon" variant="ghost" onClick={() => patch(r.id, { current: Math.max(0, r.current - 1) })} aria-label="Spend">−</Button>
+              <Button size="icon" variant="ghost" onClick={() => { const res = spendResource(c, r.id); if (res.ok) update(res.patch); }} aria-label="Spend">−</Button>
               <span className="w-12 text-center text-sm">
                 <span className="font-medium text-ink">{r.current}</span>
                 <span className="text-muted">/{r.max}</span>
               </span>
-              <Button size="icon" variant="ghost" onClick={() => patch(r.id, { current: Math.min(r.max, r.current + 1) })} aria-label="Regain">+</Button>
+              <Button size="icon" variant="ghost" onClick={() => { const res = regainResource(c, r.id); if (res.ok) update(res.patch); }} aria-label="Regain">+</Button>
               <NumberField className="w-14" value={r.max} min={0} onChange={(v) => patch(r.id, { max: v, current: Math.min(v, r.current) })} aria-label="Max" />
               <Select className="w-auto py-1 text-xs" value={r.recovery} onChange={(e) => patch(r.id, { recovery: e.target.value as CharacterResource['recovery'] })}>
                 {(['short', 'long', 'daily', 'none'] as const).map((k) => (

src/features/characters/sheet/SpellcastingSection.tsx

@@ -1,8 +1,10 @@
 import { useState } from 'react';
+import { Sparkles, BrainCircuit } from 'lucide-react';
 import { newId } from '@/lib/ids';
 import { getSystem } from '@/lib/rules';
 import type { AbilityKey, CharacterRulesInput, ProficiencyRank } from '@/lib/rules';
 import { type SpellEntry, newSpellEntry } from '@/lib/schemas';
+import { castSpell, dropConcentration } from '@/lib/mechanics';
 import { formatModifier } from '@/lib/format';
 import { Button } from '@/components/ui/Button';
 import { Input, Select } from '@/components/ui/Input';
@@ -50,6 +52,18 @@ export function SpellcastingSection({ c, update }: SectionProps) {
   const removeSpell = (id: string) =>
     update({ spellcasting: { ...sc, spells: sc.spells.filter((s) => s.id !== id) } });
 
+  // Casting a spell enforces slot consumption + concentration via the kernel.
+  const [castMsg, setCastMsg] = useState<{ text: string; ok: boolean } | null>(null);
+  const cast = (id: string) => {
+    const res = castSpell(c, id);
+    if (res.ok) {
+      update(res.patch);
+      setCastMsg({ text: res.log.join(' '), ok: true });
+    } else {
+      setCastMsg({ text: res.reason, ok: false });
+    }
+  };
+
   const spellsByLevel = [...sc.spells].sort((a, b) => a.level - b.level || a.name.localeCompare(b.name));
 
   return (
@@ -76,6 +90,18 @@ export function SpellcastingSection({ c, update }: SectionProps) {
         )}
       </div>
 
+      {/* Concentration banner — enforced: one spell at a time. */}
+      {c.concentration && (
+        <div className="mb-3 flex items-center gap-2 rounded-md border border-accent/40 bg-accent/5 px-3 py-1.5 text-sm">
+          <BrainCircuit size={15} className="shrink-0 text-accent" aria-hidden />
+          <span className="text-ink">Concentrating on <span className="font-medium">{c.concentration.spellName}</span></span>
+          <Button size="sm" variant="ghost" className="ml-auto" onClick={() => update(dropConcentration(c))}>Drop</Button>
+        </div>
+      )}
+      {castMsg && (
+        <p className={`mb-3 text-xs ${castMsg.ok ? 'text-success' : 'text-danger'}`} aria-live="polite">{castMsg.text}</p>
+      )}
+
       {/* Spell slots */}
       <div className="mb-4">
         <div className="mb-1 flex items-center gap-2">
@@ -124,13 +150,19 @@ export function SpellcastingSection({ c, update }: SectionProps) {
           {spellsByLevel.map((s) => (
             <li key={s.id} className="flex items-center gap-2 rounded-md border border-line bg-panel px-3 py-1.5 text-sm">
               <span className="w-16 text-xs text-muted">{s.level === 0 ? 'Cantrip' : `Lv ${s.level}`}</span>
-              <span className="flex-1 text-ink">{s.name}</span>
+              <span className="flex-1 text-ink">
+                {s.name}
+                {s.concentration && <BrainCircuit size={12} className="ml-1 inline text-accent" aria-label="Concentration" />}
+              </span>
               {s.level > 0 && (
                 <label className="flex items-center gap-1 text-xs text-muted">
                   <input type="checkbox" checked={s.prepared} onChange={(e) => patchSpell(s.id, { prepared: e.target.checked })} />
                   Prepared
                 </label>
               )}
+              <Button size="sm" variant="ghost" onClick={() => cast(s.id)} aria-label={`Cast ${s.name}`}>
+                <Sparkles size={13} aria-hidden /> Cast
+              </Button>
               <Button size="icon" variant="ghost" className="text-danger" onClick={() => removeSpell(s.id)} aria-label={`Remove ${s.name}`}>✕</Button>
             </li>
           ))}