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/computed-stats.ts

@@ -78,7 +78,7 @@ export function getFloorMaxHP(floor: number): number {
 }
 
 export function getFloorElement(floor: number): string {
-  return FLOOR_ELEM_CYCLE[(floor - 1) % 8];
+  return FLOOR_ELEM_CYCLE[(floor - 1) % FLOOR_ELEM_CYCLE.length];
 }
 
 // ─── Equipment Spell Helper ─────────────────────────────────────────────────────