Implement the 025-display-initiative feature that adds initiative modifier and passive initiative display to creature stat blocks, calculated as DEX modifier + (proficiency multiplier × proficiency bonus) from bestiary data, shown in MM 2024 format on the AC line

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

specs/025-display-initiative/data-model.md

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+# Data Model: Display Initiative
+
+## Modified Entities
+
+### Creature (domain)
+
+**File**: `packages/domain/src/creature-types.ts`
+
+Add one new field to the `Creature` interface:
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `initiativeProficiency` | `number` | Proficiency multiplier for initiative (0, 1, or 2). Defaults to 0 when absent from bestiary data. |
+
+This field stores the raw multiplier from bestiary data. The actual initiative modifier is derived via the `calculateInitiative` function — not stored on the entity.
+
+### Initiative (derived value)
+
+**File**: `packages/domain/src/initiative.ts`
+
+Not a persisted entity — a computed result from a pure function.
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `modifier` | `number` | The initiative modifier: DEX mod + (proficiency multiplier × proficiency bonus) |
+| `passive` | `number` | The passive initiative: 10 + modifier |
+
+## New Functions
+
+### `calculateInitiative` (domain)
+
+**File**: `packages/domain/src/initiative.ts`
+
+```
+Input:  { dexScore: number, cr: string, initiativeProficiency: number }
+Output: { modifier: number, passive: number }
+```
+
+Pure function. No side effects. Uses existing `proficiencyBonus(cr)` helper.
+
+### `formatInitiativeModifier` (domain)
+
+**File**: `packages/domain/src/initiative.ts`
+
+```
+Input:  modifier: number
+Output: string (e.g., "+7", "−1", "+0")
+```
+
+Formats the modifier with explicit sign. Uses U+2212 (−) for negative values.
+
+## Modified Adapters
+
+### Bestiary Adapter
+
+**File**: `apps/web/src/adapters/bestiary-adapter.ts`
+
+- Add `initiative?: { proficiency?: number }` to `RawMonster` interface
+- Parse `m.initiative?.proficiency ?? 0` into `Creature.initiativeProficiency`
+
+## State Transitions
+
+None. This feature is purely derived/display. No state mutations, no events, no persistence changes.