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>

packages/domain/src/initiative.ts

@@ -0,0 +1,30 @@
+import { proficiencyBonus } from "./creature-types.js";
+
+export interface InitiativeResult {
+	readonly modifier: number;
+	readonly passive: number;
+}
+
+/**
+ * Calculates initiative modifier and passive initiative from creature stats.
+ * Returns undefined for combatants without bestiary creature data.
+ */
+export function calculateInitiative(creature: {
+	readonly dexScore: number;
+	readonly cr: string;
+	readonly initiativeProficiency: number;
+}): InitiativeResult {
+	const dexMod = Math.floor((creature.dexScore - 10) / 2);
+	const pb = proficiencyBonus(creature.cr);
+	const modifier = dexMod + (creature.initiativeProficiency ?? 0) * pb;
+	return { modifier, passive: 10 + modifier };
+}
+
+/**
+ * Formats an initiative modifier with explicit sign.
+ * Uses U+2212 (−) for negative values.
+ */
+export function formatInitiativeModifier(modifier: number): string {
+	if (modifier >= 0) return `+${modifier}`;
+	return `\u2212${Math.abs(modifier)}`;
+}