Implement the 002-add-combatant feature that adds the possibility to add new combatants to an encounter

specs/002-add-combatant/plan.md

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+# Implementation Plan: Add Combatant
+
+**Branch**: `002-add-combatant` | **Date**: 2026-03-03 | **Spec**: [spec.md](./spec.md)
+**Input**: Feature specification from `/specs/002-add-combatant/spec.md`
+
+## Summary
+
+Add a pure domain function `addCombatant` that appends a new combatant to the end of an encounter's combatant list without altering the active turn or round. The feature follows the same pattern as `advanceTurn`: a pure function returning updated state plus domain events, with an application-layer use case and a React adapter hook.
+
+## Technical Context
+
+**Language/Version**: TypeScript 5.x (strict mode, verbatimModuleSyntax)
+**Primary Dependencies**: None for domain; React 19 for web adapter
+**Storage**: In-memory (React state via hook)
+**Testing**: Vitest
+**Target Platform**: Browser (Vite dev server)
+**Project Type**: Monorepo (pnpm workspaces): domain library + application library + web app
+**Performance Goals**: N/A (pure synchronous function)
+**Constraints**: Domain must remain pure — no I/O, no randomness
+**Scale/Scope**: Single-user local app
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Notes |
+|-----------|--------|-------|
+| I. Deterministic Domain Core | PASS | `addCombatant` is a pure function. CombatantId is passed in as input, not generated internally. |
+| II. Layered Architecture | PASS | Domain function in `packages/domain`, use case in `packages/application`, hook in `apps/web`. No reverse imports. |
+| III. Agent Boundary | PASS | No agent layer involvement in this feature. |
+| IV. Clarification-First | PASS | Spec has no NEEDS CLARIFICATION markers. Key assumption (id passed in) is documented. |
+| V. Escalation Gates | PASS | Implementation stays within spec scope. |
+| VI. MVP Baseline Language | PASS | Out-of-scope items use "MVP baseline does not include". |
+| VII. No Gameplay Rules | PASS | No gameplay mechanics in constitution. |
+
+All gates pass. No violations to justify.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/002-add-combatant/
+├── plan.md              # This file
+├── research.md          # Phase 0 output
+├── data-model.md        # Phase 1 output
+├── quickstart.md        # Phase 1 output
+└── tasks.md             # Phase 2 output (via /speckit.tasks)
+```
+
+### Source Code (repository root)
+
+```text
+packages/domain/src/
+├── types.ts             # Encounter, Combatant, CombatantId (existing)
+├── events.ts            # DomainEvent union (add CombatantAdded)
+├── add-combatant.ts     # NEW: addCombatant pure function
+├── advance-turn.ts      # Existing (unchanged)
+├── index.ts             # Re-exports (add new exports)
+└── __tests__/
+    ├── advance-turn.test.ts  # Existing (unchanged)
+    └── add-combatant.test.ts # NEW: acceptance + invariant tests
+
+packages/application/src/
+├── ports.ts                      # EncounterStore (unchanged)
+├── add-combatant-use-case.ts     # NEW: orchestrates addCombatant
+├── advance-turn-use-case.ts      # Existing (unchanged)
+└── index.ts                      # Re-exports (add new exports)
+
+apps/web/src/
+├── App.tsx              # Update: add combatant input + button
+└── hooks/
+    └── use-encounter.ts # Update: expose addCombatant callback
+```
+
+**Structure Decision**: Follows the established monorepo layout. Each domain operation gets its own file (matching `advance-turn.ts` pattern). No new packages or directories needed beyond the existing structure.