Add encounter difficulty indicator (5.5e XP budget)

Live 3-bar difficulty indicator in the top bar showing encounter
difficulty (Trivial/Low/Moderate/High) based on the 2024 5.5e XP
budget system. Automatically derived from PC levels and bestiary
creature CRs.

- Add optional level field (1-20) to PlayerCharacter
- Add CR-to-XP and XP Budget per Character lookup tables in domain
- Add calculateEncounterDifficulty pure function
- Add DifficultyIndicator component with color-coded bars and tooltip
- Add useDifficulty hook composing encounter, PC, and bestiary contexts
- Indicator hidden when no PCs with levels or no bestiary-linked monsters
- Level field in PC create/edit forms, persisted in storage

Closes #18

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

specs/008-encounter-difficulty/quickstart.md

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+# Quickstart: Encounter Difficulty Indicator
+
+**Date**: 2026-03-27 | **Feature**: 008-encounter-difficulty
+
+## Implementation Order
+
+### Phase 1: Domain — Level field + validation
+
+1. Add `level?: number` to `PlayerCharacter` in `player-character-types.ts`
+2. Add level validation to `createPlayerCharacter()` — validate if provided: integer, 1-20
+3. Add level validation to `editPlayerCharacter()` — same rules in `validateFields()`, apply in `applyFields()`
+4. Add tests for level validation in existing test files
+5. Export updated types from `index.ts`
+
+### Phase 2: Domain — Difficulty calculation
+
+1. Create `encounter-difficulty.ts` with:
+   - `CR_TO_XP` lookup (Record<string, number>)
+   - `XP_BUDGET_PER_CHARACTER` lookup (Record<number, { low, moderate, high }>)
+   - `crToXp(cr: string): number` — returns 0 for unknown CRs
+   - `calculateEncounterDifficulty(partyLevels: number[], monsterCrs: string[]): DifficultyResult`
+   - `DifficultyTier` type and `DifficultyResult` type
+2. Add comprehensive unit tests covering:
+   - All CR string formats (0, 1/8, 1/4, 1/2, integers)
+   - All difficulty tiers including trivial
+   - DMG example encounters (from issue comments)
+   - Edge cases: empty arrays, unknown CRs, mixed levels
+3. Export from `index.ts`
+
+### Phase 3: Application — Pass level through use cases
+
+1. Update `CreatePlayerCharacterUseCase` to accept and pass `level`
+2. Update `EditPlayerCharacterUseCase` to accept and pass `level`
+
+### Phase 4: Web — Level field in PC forms
+
+1. Update player characters context to pass `level` in create/edit calls
+2. Add level input field to create player modal (optional number, 1-20)
+3. Add level display + edit in player character manager
+4. Test: create PC with level, edit level, verify persistence
+
+### Phase 5: Web — Difficulty indicator
+
+1. Create `useDifficulty()` hook:
+   - Consume encounter context, player characters context, bestiary hook
+   - Map combatants → party levels + monster CRs
+   - Call domain `calculateEncounterDifficulty()`
+   - Return `DifficultyResult | null` (null when insufficient data)
+2. Create `DifficultyIndicator` component:
+   - Render 3 bars with conditional fill colors
+   - Add `title` attribute for tooltip
+   - Hidden when hook returns null
+3. Add indicator to `TurnNavigation` component, right of active combatant name
+4. Test: manual verification with various encounter compositions
+
+## Key Patterns to Follow
+
+- **Domain purity**: `calculateEncounterDifficulty` takes `number[]` and `string[]`, not domain types
+- **Validation pattern**: Follow `color`/`icon` optional field pattern in create/edit
+- **Hook composition**: `useDifficulty` composes multiple contexts like `useInitiativeRolls`
+- **Component size**: DifficultyIndicator should be <8 props (likely 0-1, just the result)
+
+## Testing Strategy
+
+- **Domain tests** (unit): Exhaustive coverage of `calculateEncounterDifficulty` and `crToXp` with table-driven tests. Cover all 34 CR values, all 20 levels, and the DMG example encounters.
+- **Domain tests** (level validation): Test create/edit with valid levels, invalid levels, and undefined level.
+- **Integration**: Verify indicator appears/hides correctly through component rendering (if existing test patterns support this).