V2 Phase 1: mechanics kernel + compendium→sheet integration

Lay the foundation for full rules enforcement: a new pure, testable
mechanics kernel (src/lib/mechanics/) that derives structured, enforceable
game data from the loosely-typed compendium, plus the first user-facing
payoff (accurate armor AC + spell mechanics on the sheet).

- normalizers: spell (5e + pf2e), armor, monster defenses → typed
  SpellMechanics / ArmorMechanics / DamageDefenses, with memoized derivers
  wrapping the existing lazy compendium loaders (no new assets). 11 unit tests.
- character schema: spell entries snapshot concentration/save/damage at
  compendium-link time; new top-level `concentration` slot and structured
  `equippedArmor` (additive, defaulted). Dexie v14 migration backfills.
- AC model: baseArmorClass now consults equippedArmor (heavy = flat, medium =
  Dex-capped) and falls back to the old 10+Dex formula when unarmored. Threaded
  through every AC call site; armor picker added to the sheet (5e).
- compendium "Add spell to character" snapshots real mechanics via the kernel.

Backward-compatible: un-enriched spells/characters behave exactly as before.
Build + 244 unit tests green; armor→AC verified live in preview.

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

src/features/characters/CharacterSheet.tsx

@@ -1,9 +1,10 @@
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 import { Link } from '@tanstack/react-router';
 import { ArrowLeft, Check, Shield, Gauge, Footprints, Award } from 'lucide-react';
 import type { Character } from '@/lib/schemas';
 import type { AbilityKey, CharacterRulesInput, ProficiencyRank, SystemId } from '@/lib/rules';
 import { getSystem, ABILITY_ABBR } from '@/lib/rules';
+import { allArmorMechanics5e, type ArmorMechanics } from '@/lib/mechanics';
 import { charactersRepo, campaignsRepo } from '@/lib/db/repositories';
 import { encodeClaim } from '@/lib/sync/playerLink';
 import { fileToDataUrl, squareThumbnail } from '@/lib/img/resize';
@@ -99,6 +100,7 @@ export function CharacterSheet({ character }: { character: Character }) {
     skillRanks: c.skillRanks as Record<string, ProficiencyRank>,
     saveRanks: c.saveRanks as Partial<Record<AbilityKey, ProficiencyRank>>,
     armorBonus: c.armorBonus,
+    ...(c.equippedArmor ? { equippedArmor: c.equippedArmor } : {}),
     perceptionRank: c.perceptionRank,
     ...(c.spellcastingRank ? { spellcastingRank: c.spellcastingRank } : {}),
   };
@@ -184,10 +186,13 @@ export function CharacterSheet({ character }: { character: Character }) {
 
       {/* Vital stats */}
       <div className="mb-6 grid gap-3 sm:grid-cols-3">
-        <StatCard label="Armor Class" value={ac} hint={`10 + DEX${c.armorBonus ? ` + ${c.armorBonus}` : ''}`}>
-          <div className="mt-2 flex items-center justify-center gap-2 text-xs text-muted">
-            <span>Armor/shield bonus</span>
-            <NumberField className="w-16" value={c.armorBonus} onChange={(armorBonus) => update({ armorBonus })} aria-label="Armor bonus" />
+        <StatCard label="Armor Class" value={ac} hint={c.equippedArmor ? `${c.equippedArmor.name}${c.armorBonus ? ` + ${c.armorBonus}` : ''}` : `10 + DEX${c.armorBonus ? ` + ${c.armorBonus}` : ''}`}>
+          <div className="mt-2 flex flex-col items-center gap-2 text-xs text-muted">
+            {c.system === '5e' && <ArmorPicker c={c} update={update} />}
+            <div className="flex items-center gap-2">
+              <span>Shield/misc bonus</span>
+              <NumberField className="w-16" value={c.armorBonus} onChange={(armorBonus) => update({ armorBonus })} aria-label="Armor bonus" />
+            </div>
           </div>
         </StatCard>
 
@@ -428,6 +433,28 @@ function StatCard({ label, value, hint, children }: { label: string; value: stri
   );
 }
 
+function ArmorPicker({ c, update }: { c: Character; update: (p: Partial<Character>) => void }) {
+  const [armors, setArmors] = useState<ArmorMechanics[]>([]);
+  useEffect(() => { void allArmorMechanics5e().then(setArmors); }, []);
+  // Body armor only — shields are handled via the misc bonus field.
+  const body = armors.filter((a) => a.category !== 'shield');
+  return (
+    <Select
+      className="w-auto py-1 text-xs"
+      aria-label="Equipped armor"
+      value={c.equippedArmor?.name ?? ''}
+      onChange={(e) => {
+        const name = e.target.value;
+        const found = body.find((a) => a.name === name);
+        update({ equippedArmor: found ? { name: found.name, category: found.category, baseAc: found.baseAc, dexCap: found.dexCap, stealthDisadvantage: found.stealthDisadvantage } : null });
+      }}
+    >
+      <option value="">Unarmored</option>
+      {body.map((a) => <option key={a.name} value={a.name}>{a.name} (AC {a.baseAc}{a.dexCap === 0 ? '' : a.dexCap === null ? ' + Dex' : ` + Dex≤${a.dexCap}`})</option>)}
+    </Select>
+  );
+}
+
 function SectionTitle({ children }: { children: React.ReactNode }) {
   return <h2 className="mb-3 font-display text-lg font-semibold text-ink">{children}</h2>;
 }