Add advantage/disadvantage rolling for initiative

Right-click or long-press the d20 button (per-combatant or Roll All)
to open a context menu with Advantage and Disadvantage options.
Normal left-click behavior is unchanged.

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

packages/domain/src/__tests__/roll-initiative.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { rollInitiative } from "../roll-initiative.js";
+import { rollInitiative, selectRoll } from "../roll-initiative.js";
 import { isDomainError } from "../types.js";
 import { expectDomainError } from "./test-helpers.js";
 
@@ -63,3 +63,31 @@ describe("rollInitiative", () => {
 		});
 	});
 });
+
+describe("selectRoll", () => {
+	it("normal mode returns the first roll", () => {
+		expect(selectRoll(8, 15, "normal")).toBe(8);
+	});
+
+	it("advantage returns the higher roll", () => {
+		expect(selectRoll(8, 15, "advantage")).toBe(15);
+	});
+
+	it("advantage returns the higher roll (reversed)", () => {
+		expect(selectRoll(15, 8, "advantage")).toBe(15);
+	});
+
+	it("disadvantage returns the lower roll", () => {
+		expect(selectRoll(8, 15, "disadvantage")).toBe(8);
+	});
+
+	it("disadvantage returns the lower roll (reversed)", () => {
+		expect(selectRoll(15, 8, "disadvantage")).toBe(8);
+	});
+
+	it("equal rolls return the same value for all modes", () => {
+		expect(selectRoll(12, 12, "normal")).toBe(12);
+		expect(selectRoll(12, 12, "advantage")).toBe(12);
+		expect(selectRoll(12, 12, "disadvantage")).toBe(12);
+	});
+});

packages/domain/src/index.ts

@@ -84,7 +84,11 @@ export {
 	removeCombatant,
 } from "./remove-combatant.js";
 export { retreatTurn } from "./retreat-turn.js";
-export { rollInitiative } from "./roll-initiative.js";
+export {
+	type RollMode,
+	rollInitiative,
+	selectRoll,
+} from "./roll-initiative.js";
 export { type SetAcSuccess, setAc } from "./set-ac.js";
 export { type SetHpSuccess, setHp } from "./set-hp.js";
 export {

packages/domain/src/roll-initiative.ts

@@ -1,5 +1,21 @@
 import type { DomainError } from "./types.js";
 
+export type RollMode = "normal" | "advantage" | "disadvantage";
+
+/**
+ * Selects the effective roll from two dice values based on the roll mode.
+ * Advantage takes the higher, disadvantage takes the lower.
+ */
+export function selectRoll(
+	roll1: number,
+	roll2: number,
+	mode: RollMode,
+): number {
+	if (mode === "advantage") return Math.max(roll1, roll2);
+	if (mode === "disadvantage") return Math.min(roll1, roll2);
+	return roll1;
+}
+
 /**
  * Pure function that computes initiative from a resolved dice roll and modifier.
  * The dice roll must be an integer in [1, 20].