Add manual CR assignment and difficulty breakdown panel

Implement issue #21: custom combatants can now have a challenge rating
assigned via a new breakdown panel, opened by tapping the difficulty
indicator. Bestiary-linked combatants show read-only CR with source name;
custom combatants get a CR picker with all standard 5e values. CR persists
across reloads and round-trips through JSON export/import.

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

packages/domain/src/events.ts

@@ -94,6 +94,13 @@ export interface AcSet {
 	readonly newAc: number | undefined;
 }
 
+export interface CrSet {
+	readonly type: "CrSet";
+	readonly combatantId: CombatantId;
+	readonly previousCr: string | undefined;
+	readonly newCr: string | undefined;
+}
+
 export interface ConditionAdded {
 	readonly type: "ConditionAdded";
 	readonly combatantId: CombatantId;
@@ -153,6 +160,7 @@ export type DomainEvent =
 	| TurnRetreated
 	| RoundRetreated
 	| AcSet
+	| CrSet
 	| ConditionAdded
 	| ConditionRemoved
 	| ConcentrationStarted