fix: Spire tab maxFloorReached undefined error

docs/context-grimoire-tab-fix.md

@@ -0,0 +1,37 @@
+# Context: Grimoire/Spells Tab "cost.map" Error Fix
+
+## Problem Statement
+The Grimoire/Spells tab fails to load with error: `TypeError: e.cost.map is not a function`
+
+## Error Analysis
+This error indicates that `e.cost` is not an array (or is undefined) when `.map()` is called. Likely causes:
+1. Spell cost is defined as a single value instead of an array
+2. Missing or malformed spell definitions
+3. Incorrect data structure in spells constants
+4. Legacy store returning incorrect spell state
+
+## Key Files to Investigate/Modify
+- `src/components/game/tabs/SpellsTab.tsx` - Spells tab component (likely "Grimoire" tab)
+- `src/lib/game/constants/spells.ts` - Spell definitions
+- `src/lib/game/constants/spells-modules/` - Modular spell definitions
+- `src/lib/game/stores/skillStore.ts` - Skill state (affects spell unlocking)
+- `src/lib/game/stores/combatStore.ts` - Spell state
+
+## Architecture Rules (from AGENTS.md)
+- Use modular stores: import from `src/lib/game/stores/`
+- Spell definitions belong in `src/lib/game/constants/spells.ts` or `spells-modules/`
+- All files must stay under 400 lines
+- No legacy store references
+
+## Debugging Steps
+1. Find where `.map()` is called on spell cost in SpellsTab.tsx
+2. Check spell definition structure for `cost` field
+3. Verify spell cost is always an array (even for single cost)
+4. Check if cost is properly initialized for all spells
+5. Ensure spells are correctly loaded from constants
+
+## Expected Outcome
+- Spells tab loads without errors
+- All spell costs are properly formatted as arrays
+- Spell definitions are consistent and valid
+- Regression test added to `src/lib/game/stores/__tests__/index-tests/spell-cost.test.ts`