Phase 3: soft level pressure, less experience far above a zone's band

- Companion mod (zone/LevelPressure.kt): a Pokemon more than 5 levels
  above the band of the zone its trainer is in gets 10% less experience
  per extra level, down to 10%. Exp. Candy and commands are exempt; no
  hard level caps (roadmap).
- Rules in world/zones.json -> level_pressure, shipped in the zone layout.
- Self-test level_pressure_cuts_xp.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/zone_layout.py

@@ -312,6 +312,7 @@ def write_datapack(zone, plan, preview):
         "seed": preview["seed"], "radius": preview["radius"], "step": preview["step"], "size": preview["size"],
         "zones": [{k: z[k] for k in keep} for z in plan["zones"]],
         "caves": [{**{k: c[k] for k in keep}, "biome": c["biome"]} for c in plan["caves"]],
+        "level_pressure": plan.get("level_pressure", {}),
         "samples": samples(zone, plan, preview),
         "biome_samples": biome_samples(zone, plan, preview),
         "grid_rle": runs,