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/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-04T09:37:32.833Z",
+    "generated": "2026-06-04T11:37:49.892Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },