Implement the 016-combatant-ac feature that adds an optional Armor Class field to combatants with shield icon display and inline editing in the encounter tracker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/016-combatant-ac/quickstart.md

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+# Quickstart: Combatant Armor Class Display
+
+**Feature**: 016-combatant-ac | **Date**: 2026-03-06
+
+## Overview
+
+This feature adds an optional Armor Class (AC) field to combatants, displayed as a shield icon with the numeric value in the encounter list. It follows the exact patterns established by initiative and HP fields.
+
+## Layer-by-Layer Implementation Order
+
+### 1. Domain Layer (`packages/domain/src/`)
+
+**Pattern to follow**: `set-initiative.ts` (single optional numeric field)
+
+1. Add `readonly ac?: number` to `Combatant` interface in `types.ts`
+2. Create `set-ac.ts` with a `setAc` pure function:
+   - Find combatant by ID (error if not found)
+   - Validate AC is non-negative integer when defined
+   - Return updated encounter + `AcSet` event
+3. Add `AcSet` event to `events.ts` and the `DomainEvent` union
+4. Export from `index.ts`
+5. Write tests in `__tests__/set-ac.test.ts`
+
+### 2. Application Layer (`packages/application/src/`)
+
+**Pattern to follow**: `set-initiative-use-case.ts`
+
+1. Create `set-ac-use-case.ts`: get encounter from store, call `setAc`, save result
+2. Export from `index.ts`
+
+### 3. Web Layer (`apps/web/src/`)
+
+**Pattern to follow**: Initiative input in `combatant-row.tsx` + `MaxHpInput` editing pattern
+
+1. **Persistence** (`persistence/encounter-storage.ts`): Add AC validation in rehydration logic
+2. **Hook** (`hooks/use-encounter.ts`): Add `setAc` callback using `setAcUseCase`
+3. **UI** (`components/combatant-row.tsx`):
+   - Add `onSetAc` to `CombatantRowProps`
+   - Add AC display: Lucide `Shield` icon + value, between name and HP
+   - Inline editable input (same pattern as `MaxHpInput`)
+   - Hidden when AC is `undefined`
+
+## Key Files to Read First
+
+| File | Why |
+|------|-----|
+| `packages/domain/src/set-initiative.ts` | Closest pattern for single optional field |
+| `packages/domain/src/events.ts` | Event type definitions |
+| `apps/web/src/components/combatant-row.tsx` | UI layout and inline input patterns |
+| `apps/web/src/persistence/encounter-storage.ts` | Rehydration validation pattern |
+
+## Testing Strategy
+
+- **Domain**: Pure function tests for `setAc` (set, clear, validation errors, combatant not found)
+- **Persistence**: Round-trip test for AC field in localStorage
+- **No UI tests**: Consistent with existing approach (no component tests in codebase)
+
+## Commands
+
+```bash
+pnpm test                    # Run all tests
+pnpm vitest run packages/domain/src/__tests__/set-ac.test.ts  # Run AC domain tests
+pnpm check                   # Full merge gate (must pass before commit)
+```