Implement the 026-roll-initiative feature that adds d20 roll buttons for bestiary combatants' initiative using a click-to-edit pattern (d20 icon when empty, plain text when set), plus a Roll All button in the top bar that batch-rolls for all unrolled bestiary combatants, with randomness confined to the adapter layer and the domain receiving pre-resolved dice values

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

packages/domain/src/roll-initiative.ts

@@ -0,0 +1,21 @@
+import type { DomainError } from "./types.js";
+
+/**
+ * Pure function that computes initiative from a resolved dice roll and modifier.
+ * The dice roll must be an integer in [1, 20].
+ * Returns the sum (diceRoll + modifier) or a DomainError for invalid inputs.
+ */
+export function rollInitiative(
+	diceRoll: number,
+	modifier: number,
+): number | DomainError {
+	if (!Number.isInteger(diceRoll) || diceRoll < 1 || diceRoll > 20) {
+		return {
+			kind: "domain-error",
+			code: "invalid-dice-roll",
+			message: `Dice roll must be an integer between 1 and 20, got ${diceRoll}`,
+		};
+	}
+
+	return diceRoll + modifier;
+}