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>

world/zones.json

@@ -1,8 +1,9 @@
 {
-  "_comment": "Zone plan for Phase 3 (model approved by Adrian 2026-10-29; levels are placeholders, see docs/balance.md). tools/zone_layout.py assigns each surface cell of the island to the first zone whose rule matches, in this order, and renders build/zone-map.png. Rules: 'within' = distance from 0,0 in blocks; 'biomes' = cells of these biomes, clusters smaller than min_cells dropped, plus 'absorb' = cells of other biomes within that many blocks of the cluster; 'region' = what is left, by half-plane (x/z bounds in blocks); 'water' = sea cells of these biomes. Biomes in 'inherit' (rivers) take the zone of the land around them. Cave zones follow the cave biome everywhere under the island and are not drawn. 'lines' = how many evolution lines tools/zone_spawns.py aims to give the zone (scaled to the lines available); 'species' pins a line to a zone by hand. 'guardian': the zone's Alpha guardian (tools/zone_spawns.py picks the strongest Pokémon spawning in the zone unless 'species' is given, at the band's top level + 5) and its drops, which help with the next zone; it stands at the zone's sample point and returns guardian_respawn_hours after it is beaten (Adrian, 2026-10-29: fixed spots, 5 hours).",
+  "_comment": "Zone plan for Phase 3 (model approved by Adrian 2026-10-29; levels are placeholders, see docs/balance.md). tools/zone_layout.py assigns each surface cell of the island to the first zone whose rule matches, in this order, and renders build/zone-map.png. Rules: 'within' = distance from 0,0 in blocks; 'biomes' = cells of these biomes, clusters smaller than min_cells dropped, plus 'absorb' = cells of other biomes within that many blocks of the cluster; 'region' = what is left, by half-plane (x/z bounds in blocks); 'water' = sea cells of these biomes. Biomes in 'inherit' (rivers) take the zone of the land around them. Cave zones follow the cave biome everywhere under the island and are not drawn. 'lines' = how many evolution lines tools/zone_spawns.py aims to give the zone (scaled to the lines available); 'species' pins a line to a zone by hand. 'guardian': the zone's Alpha guardian (tools/zone_spawns.py picks the strongest Pokémon spawning in the zone unless 'species' is given, at the band's top level + 5) and its drops, which help with the next zone; it stands at the zone's sample point and returns guardian_respawn_hours after it is beaten (Adrian, 2026-10-29: fixed spots, 5 hours). 'level_pressure': a Pokémon more than grace_levels above the band of the zone its trainer stands in gets per_level less experience per extra level, down to minimum (Exp. Candy and commands excepted).",
   "min_cells": 6,
   "inherit": ["minecraft:river"],
   "guardian_respawn_hours": 5,
+  "level_pressure": {"grace_levels": 5, "per_level": 0.1, "minimum": 0.1},
   "species": {"durant": "basalt", "torkoal": "basalt", "heatmor": "basalt"},
   "zones": [
     {