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/encounter-difficulty.ts

@@ -74,6 +74,9 @@ const XP_BUDGET_PER_CHARACTER: Readonly<
 	20: { low: 6400, moderate: 13200, high: 22000 },
 };
 
+/** All standard 5e challenge rating strings, in ascending order. */
+export const VALID_CR_VALUES: readonly string[] = Object.keys(CR_TO_XP);
+
 /** Returns the XP value for a given CR string. Returns 0 for unknown CRs. */
 export function crToXp(cr: string): number {
 	return CR_TO_XP[cr] ?? 0;