feat: overhaul mana conversion system to unified regen-deduction model

- New files: element-distance.ts, conversion-costs.ts, conversion-rates.ts
- All conversion types (discipline, attunement, pact) use unified formula
- Conversion costs scale exponentially by element tier (10^(d+1) raw, 10*(d+1) per component)
- Costs deducted from regen, not from mana pool
- Auto-pause on insufficient regen with UI warning
- Meditation boosts conversion rates (reduced by distance)
- Attunement levels provide +50% multiplicative bonus per level
- Guardian pacts provide +0.15/hr base rate + invoker level bonus
- Removed convertMana, processConvertAction, craftComposite from manaStore
- Stats tab shows per-element conversion breakdown with formulas
- ManaDisplay shows per-element net regen rates
- All 916 tests pass, all files under 400 lines

docs/project-structure.txt

@@ -327,6 +327,7 @@ Mana-Loop/
 │   │   │   │   │   └── utils.ts
 │   │   │   │   ├── achievements.ts
 │   │   │   │   ├── attunements.ts
+│   │   │   │   ├── conversion-costs.ts
 │   │   │   │   ├── crafting-recipes.ts
 │   │   │   │   ├── enchantment-effects.ts
 │   │   │   │   ├── enchantment-types.ts
@@ -389,8 +390,10 @@ Mana-Loop/
 │   │   │   ├── utils/
 │   │   │   │   ├── activity-log.ts
 │   │   │   │   ├── combat-utils.ts
+│   │   │   │   ├── conversion-rates.ts
 │   │   │   │   ├── discipline-math.ts
 │   │   │   │   ├── element-cap-bonus.ts
+│   │   │   │   ├── element-distance.ts
 │   │   │   │   ├── enemy-generator.ts
 │   │   │   │   ├── enemy-utils.ts
 │   │   │   │   ├── floor-utils.ts