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/stores.test.ts

@@ -261,17 +261,17 @@ describe('SkillStore', () => {
     it('should not start studying without prerequisites', () => {
       useManaStore.getState().addRawMana(990, 1000);
       
-      // deepReservoir requires manaWell 5
-      const result = useSkillStore.getState().startStudyingSkill('deepReservoir', 1000);
+      // manaOverflow requires manaWell 3
+      const result = useSkillStore.getState().startStudyingSkill('manaOverflow', 1000);
       
       expect(result.started).toBe(false);
     });
     
     it('should start studying with prerequisites met', () => {
       useManaStore.getState().addRawMana(990, 1000);
-      useSkillStore.getState().setSkillLevel('manaWell', 5);
+      useSkillStore.getState().setSkillLevel('manaWell', 3);
       
-      const result = useSkillStore.getState().startStudyingSkill('deepReservoir', 1000);
+      const result = useSkillStore.getState().startStudyingSkill('manaOverflow', 1000);
       
       expect(result.started).toBe(true);
     });