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/plan.md

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+# Implementation Plan: Combatant Armor Class Display
+
+**Branch**: `016-combatant-ac` | **Date**: 2026-03-06 | **Spec**: [spec.md](./spec.md)
+**Input**: Feature specification from `/specs/016-combatant-ac/spec.md`
+
+## Summary
+
+Add an optional Armor Class (AC) field to the Combatant domain type and display it in the encounter list as a shield icon with the numeric value next to each combatant's name. The feature follows the exact same patterns as existing optional fields (initiative, maxHp) through all layers: domain pure function, application use case, React UI component, and localStorage persistence.
+
+## Technical Context
+
+**Language/Version**: TypeScript 5.8 (strict mode, verbatimModuleSyntax)
+**Primary Dependencies**: React 19, Vite 6, Tailwind CSS v4, shadcn/ui-style components, Lucide React (icons)
+**Storage**: Browser localStorage (existing adapter, transparent JSON serialization)
+**Testing**: Vitest (pure domain function tests + storage round-trip tests)
+**Target Platform**: Browser (local-first, single-user)
+**Project Type**: Web application (monorepo: packages/domain, packages/application, apps/web)
+**Performance Goals**: N/A (trivial data addition, no performance concerns)
+**Constraints**: Offline-capable (localStorage only), no external dependencies added
+**Scale/Scope**: Single optional field addition across 3 layers
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Evidence |
+|-----------|--------|----------|
+| I. Deterministic Domain Core | PASS | `setAc` is a pure function: same input encounter + combatantId + ac value always produces the same output. No I/O, randomness, or clocks. |
+| II. Layered Architecture | PASS | Domain defines `setAc` (pure). Application defines `setAcUseCase` (orchestration via EncounterStore port). Web implements UI + persistence adapter. No reverse dependencies. |
+| III. Agent Boundary | N/A | No agent layer involvement. |
+| IV. Clarification-First | PASS | No non-trivial assumptions — feature follows established patterns for optional combatant fields. |
+| V. Escalation Gates | PASS | All functionality is within spec scope (FR-001 through FR-007). |
+| VI. MVP Baseline Language | PASS | Spec uses "MVP baseline does not include AC-based calculations." |
+| VII. No Gameplay Rules | PASS | AC is stored and displayed only — no combat resolution logic. |
+
+**Gate result**: PASS — no violations.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/016-combatant-ac/
+├── plan.md
+├── research.md
+├── data-model.md
+├── quickstart.md
+└── tasks.md              # Phase 2 output (/speckit.tasks)
+```
+
+### Source Code (repository root)
+
+```text
+packages/domain/src/
+├── types.ts              # Add optional `ac` field to Combatant interface
+├── set-ac.ts             # NEW: pure function to set/clear AC
+├── events.ts             # Add AcSet event type
+├── index.ts              # Re-export new function + event type
+└── __tests__/
+    └── set-ac.test.ts    # NEW: domain tests for setAc
+
+packages/application/src/
+├── set-ac-use-case.ts    # NEW: orchestration use case
+└── index.ts              # Re-export new use case
+
+apps/web/src/
+├── components/
+│   └── combatant-row.tsx # Add AC display (shield icon + value) + onSetAc callback
+├── hooks/
+│   └── use-encounter.ts  # Add setAc action callback
+└── persistence/
+    └── encounter-storage.ts  # Add AC validation in loadEncounter rehydration
+    └── __tests__/
+        └── encounter-storage.test.ts  # Add AC round-trip tests
+```
+
+**Structure Decision**: Follows the existing monorepo layered architecture (domain → application → web). No new packages or structural changes needed.
+
+## Complexity Tracking
+
+> No violations — table not applicable.