refactor: resolve structural inconsistencies and dead code

- Fix broken barrel exports in components/game/index.ts
- Remove skill system from stores (gameStore, gameActions, gameLoopActions, gameHooks, craftingStore, combat)
- Remove skill system from components (page.tsx, LeftPanel, StatsTab, SpellsTab, EnchantmentDesigner, EnchantmentPreparer, GameContext/Provider)
- Delete dead code: stats/ directory, attunements/ directory, layout/ Header+TabBar, shared/ StudyProgress+UpgradeDialog duplicates, effects.ts.fix, study-slice.ts, navigation-slice.ts
- Delete legacy store/ and store-modules/ directories, redirect remaining callers
- Merge root formatting.ts into utils/formatting.ts
- Move effects files (dynamic-compute, upgrade-effects, special-effects, upgrade-effects.types) into effects/ directory
- Move debug-context.tsx into components/game/debug/
- Create tabs/index.ts barrel for tab components
- Fix page.tsx lazy imports to use tabs barrel
- Fix all broken import paths across codebase
- Remove SKILLS_DEF and skill-evolution references
- Trim store.ts to under 400 lines by removing dead skill actions

src/lib/game/effects/upgrade-effects.ts

@@ -0,0 +1,68 @@
+// ─── Upgrade Effect System ────────────────────────────────────────────────────────
+// This module handles applying skill upgrade effects to game stats
+// Note: Skill evolution paths have been removed. Upgrade effects now return
+// base/default values with no skill-dependent modifications.
+
+import type { ActiveUpgradeEffect, ComputedEffects } from './upgrade-effects.types';
+
+// ─── Upgrade Definition Cache ───────────────────────────
+
+// No-op: skill evolution paths have been removed
+function buildUpgradeCache(): void {
+  // No-op: no upgrade definitions to cache
+}
+
+// ─── Helper Functions ──────────────────────────────
+
+/**
+ * Get all selected upgrades with their full effect definitions.
+ * Since skills are removed, always returns empty array.
+ */
+export function getActiveUpgrades(
+  _skillUpgrades: Record<string, string[]>,
+  _skillTiers: Record<string, number>
+): ActiveUpgradeEffect[] {
+  return [];
+}
+
+/**
+ * Compute all active effects from selected upgrades.
+ * Since skills are removed, returns base values with no upgrades applied.
+ */
+export function computeEffects(
+  _skillUpgrades: Record<string, string[]>,
+  _skillTiers: Record<string, number>
+): ComputedEffects {
+  return {
+    maxManaMultiplier: 1,
+    maxManaBonus: 0,
+    regenMultiplier: 1,
+    regenBonus: 0,
+    clickManaMultiplier: 1,
+    clickManaBonus: 0,
+    meditationEfficiency: 1,
+    spellCostMultiplier: 1,
+    conversionEfficiency: 1,
+    baseDamageMultiplier: 1,
+    baseDamageBonus: 0,
+    attackSpeedMultiplier: 1,
+    critChanceBonus: 0,
+    critDamageMultiplier: 1.5,
+    elementalDamageMultiplier: 1,
+    studySpeedMultiplier: 1,
+    studyCostMultiplier: 1,
+    progressRetention: 0,
+    instantStudyChance: 0,
+    freeStudyChance: 0,
+    elementCapMultiplier: 1,
+    elementCapBonus: 0,
+    perElementCapBonus: {},
+    conversionCostMultiplier: 1,
+    doubleCraftChance: 0,
+    permanentRegenBonus: 0,
+    specials: new Set<string>(),
+    activeUpgrades: [],
+    skillLevelMultiplier: 1,
+    enchantmentPowerMultiplier: 1,
+  };
+}