docs: add comprehensive game briefing document and fix deprecated tests

- Created GAME_BRIEFING.md with full documentation of all game systems
- Fixed getFloorElement to use cycle length instead of hardcoded 8
- Fixed deprecated tests referencing removed elements (life, blood, wood)
- Fixed deprecated tests referencing removed skills (deepReservoir, etc.)
- Fixed guardian tests to not expect floor 70
- Fixed computeRegen tests to account for attunement regen correctly
- All 512 tests now pass

The game briefing document includes:
- Core game loop and progression
- Mana system with all 14 mana types
- Time and incursion mechanics
- Spire and floor system with room types
- Combat system with elemental effectiveness
- Guardian and pact system
- Attunement system (Enchanter, Invoker, Fabricator)
- Skill evolution with 5 tiers and milestone upgrades
- Equipment and enchantment system
- Golemancy system
- Prestige/loop mechanics
- Complete formulas and system interactions

src/lib/game/__tests__/computed-stats.test.ts

@@ -221,7 +221,8 @@ describe('computeRegen', () => {
     };
     const effects = { regenBonus: 0, regenMultiplier: 1, permanentRegenBonus: 0 };
     const result = computeRegen(state, effects);
-    expect(result).toBe(2); // Base regen
+    // Base regen is 2 (this test provides effects, so no attunement bonus)
+    expect(result).toBe(2);
   });
 });