Add Pathfinder 2e game system mode

Implements PF2e as an alternative game system alongside D&D 5e/5.5e.
Settings modal "Game System" selector switches conditions, bestiary,
stat block layout, and initiative calculation between systems.

- Valued conditions with increment/decrement UX (Clumsy 2, Frightened 3)
- 2,502 PF2e creatures from bundled search index (77 sources)
- PF2e stat block: level, traits, Perception, Fort/Ref/Will, ability mods
- Perception-based initiative rolling
- System-scoped source cache (D&D and PF2e sources don't collide)
- Backwards-compatible condition rehydration (ConditionId[] → ConditionEntry[])
- Difficulty indicator hidden in PF2e mode (excluded from MVP)

Closes dostulata/initiative#19

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

apps/web/src/components/__tests__/stat-block.test.tsx

@@ -5,7 +5,7 @@ import type { Creature } from "@initiative/domain";
 import { creatureId } from "@initiative/domain";
 import { cleanup, render, screen } from "@testing-library/react";
 import { afterEach, describe, expect, it } from "vitest";
-import { StatBlock } from "../stat-block.js";
+import { DndStatBlock as StatBlock } from "../dnd-stat-block.js";
 
 afterEach(cleanup);