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/__tests__/rehydrate-combatant.test.ts

@@ -219,6 +219,28 @@ describe("rehydrateCombatant", () => {
 			}
 		});
 
+		it("preserves valid cr field", () => {
+			for (const cr of ["5", "1/4", "0", "30"]) {
+				const result = rehydrateCombatant({ ...minimalCombatant(), cr });
+				expect(result).not.toBeNull();
+				expect(result?.cr).toBe(cr);
+			}
+		});
+
+		it("drops invalid cr field", () => {
+			for (const cr of ["99", "", 42, null, "abc"]) {
+				const result = rehydrateCombatant({ ...minimalCombatant(), cr });
+				expect(result).not.toBeNull();
+				expect(result?.cr).toBeUndefined();
+			}
+		});
+
+		it("combatant without cr rehydrates as before", () => {
+			const result = rehydrateCombatant(minimalCombatant());
+			expect(result).not.toBeNull();
+			expect(result?.cr).toBeUndefined();
+		});
+
 		it("drops invalid tempHp — keeps combatant", () => {
 			for (const tempHp of [-1, 1.5, "3"]) {
 				const result = rehydrateCombatant({