Balance pass + 40-year soak test

- Power consumption x4: the grid now matters from mid-town on
- Realistic tax multipliers, trimmed service upkeep: profitable cities
- Buildings may claim empty zoned plots (services fit downtown)
- Land value ladder: maturity bonus, size-scaled upgrade sampling,
  thresholds 34/74 -> full L1->L2->L3 density progression
- Soak test: 40 simulated years, 11/11 asserts green (848 citizens,
  7 L3 towers, $190k surplus, proactive grid growth, 1.2ms avg tick)

package.json

@@ -18,7 +18,8 @@
     "preview": "vite preview",
     "test": "node tests/engine.mjs",
     "test:browser": "node tests/smoke.mjs",
-    "shots": "node tests/capture.mjs"
+    "shots": "node tests/capture.mjs",
+    "test:soak": "node tests/soak.mjs"
   },
   "dependencies": {
     "three": "^0.185.1"