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/data-model.md

@@ -0,0 +1,145 @@
+# Data Model: Encounter Difficulty Indicator
+
+**Date**: 2026-03-27 | **Feature**: 008-encounter-difficulty
+
+## Entities
+
+### PlayerCharacter (modified)
+
+Existing entity from spec 005. Adding one optional field.
+
+| Field | Type | Required | Notes |
+|-------|------|----------|-------|
+| id | PlayerCharacterId | yes | Existing — branded string |
+| name | string | yes | Existing |
+| ac | number | yes | Existing |
+| maxHp | number | yes | Existing |
+| color | PlayerColor | no | Existing |
+| icon | PlayerIcon | no | Existing |
+| **level** | **number** | **no** | **NEW — integer 1-20. Used for XP budget calculation. PCs without level are excluded from difficulty calc.** |
+
+**Validation rules for `level`**:
+- If provided, must be an integer
+- If provided, must be >= 1 and <= 20
+- If omitted/undefined, PC is excluded from difficulty budget
+
+### DifficultyTier (new)
+
+Enumeration of encounter difficulty categories.
+
+| Value | Display Label | Visual |
+|-------|---------------|--------|
+| `"trivial"` | Trivial | 3 empty bars |
+| `"low"` | Low | 1 green bar |
+| `"moderate"` | Moderate | 2 yellow bars |
+| `"high"` | High | 3 red bars |
+
+### DifficultyResult (new)
+
+Output of the difficulty calculation. Pure data object.
+
+| Field | Type | Notes |
+|-------|------|-------|
+| tier | DifficultyTier | The determined difficulty category |
+| totalMonsterXp | number | Sum of XP for all bestiary-linked combatants |
+| partyBudget | { low: number; moderate: number; high: number } | XP thresholds for the party |
+
+### XP Budget per Character (static lookup)
+
+Maps character level to XP thresholds. Data from 2024 5.5e DMG.
+
+| Level | Low | Moderate | High |
+|-------|-----|----------|------|
+| 1 | 50 | 75 | 100 |
+| 2 | 100 | 150 | 200 |
+| 3 | 150 | 225 | 400 |
+| 4 | 250 | 375 | 500 |
+| 5 | 500 | 750 | 1,100 |
+| 6 | 600 | 1,000 | 1,400 |
+| 7 | 750 | 1,300 | 1,700 |
+| 8 | 1,000 | 1,700 | 2,100 |
+| 9 | 1,300 | 2,000 | 2,600 |
+| 10 | 1,600 | 2,300 | 3,100 |
+| 11 | 1,900 | 2,900 | 4,100 |
+| 12 | 2,200 | 3,700 | 4,700 |
+| 13 | 2,600 | 4,200 | 5,400 |
+| 14 | 2,900 | 4,900 | 6,200 |
+| 15 | 3,300 | 5,400 | 7,800 |
+| 16 | 3,800 | 6,100 | 9,800 |
+| 17 | 4,500 | 7,200 | 11,700 |
+| 18 | 5,000 | 8,700 | 14,200 |
+| 19 | 5,500 | 10,700 | 17,200 |
+| 20 | 6,400 | 13,200 | 22,000 |
+
+### CR-to-XP (static lookup)
+
+Maps challenge rating strings to XP values. Standard 5e values.
+
+| CR | XP |
+|----|-----|
+| 0 | 0 |
+| 1/8 | 25 |
+| 1/4 | 50 |
+| 1/2 | 100 |
+| 1 | 200 |
+| 2 | 450 |
+| 3 | 700 |
+| 4 | 1,100 |
+| 5 | 1,800 |
+| 6 | 2,300 |
+| 7 | 2,900 |
+| 8 | 3,900 |
+| 9 | 5,000 |
+| 10 | 5,900 |
+| 11 | 7,200 |
+| 12 | 8,400 |
+| 13 | 10,000 |
+| 14 | 11,500 |
+| 15 | 13,000 |
+| 16 | 15,000 |
+| 17 | 18,000 |
+| 18 | 20,000 |
+| 19 | 22,000 |
+| 20 | 25,000 |
+| 21 | 33,000 |
+| 22 | 41,000 |
+| 23 | 50,000 |
+| 24 | 62,000 |
+| 25 | 75,000 |
+| 26 | 90,000 |
+| 27 | 105,000 |
+| 28 | 120,000 |
+| 29 | 135,000 |
+| 30 | 155,000 |
+
+## Relationships
+
+```
+PlayerCharacter (has optional level)
+    │
+    ▼ linked via playerCharacterId
+Combatant (in Encounter)
+    │
+    ▼ linked via creatureId
+Creature (has cr string)
+    │
+    ▼ lookup via CR_TO_XP table
+XP value (number)
+
+Party levels ──► XP_BUDGET_TABLE ──► { low, moderate, high } thresholds
+Monster XP total ──► compare against thresholds ──► DifficultyTier
+```
+
+## State Transitions
+
+The difficulty calculation is stateless — it's a pure derivation from current encounter state. No state machine or transitions to model.
+
+**Input derivation** (at adapter layer):
+1. For each combatant with `playerCharacterId` → look up `PlayerCharacter.level` → collect non-undefined levels
+2. For each combatant with `creatureId` → look up `Creature.cr` → collect CR strings
+3. Pass `(levels[], crs[])` to domain function
+
+**Pure calculation** (domain layer):
+1. Sum XP budget per level → `partyBudget.{low, moderate, high}`
+2. Convert each CR to XP → sum → `totalMonsterXp`
+3. Compare `totalMonsterXp` against thresholds → `DifficultyTier`