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>

mod/src/main/kotlin/dev/cobblesurvival/CobbleSurvival.kt

@@ -8,6 +8,7 @@ import dev.cobblesurvival.permadeath.Permadeath
 import dev.cobblesurvival.selftest.SelfTest
 import dev.cobblesurvival.world.OnlyPokemon
 import dev.cobblesurvival.zone.Guardians
+import dev.cobblesurvival.zone.LevelPressure
 import dev.cobblesurvival.zone.ZoneCondition
 import dev.cobblesurvival.zone.ZoneHazards
 import dev.cobblesurvival.zone.Zones
@@ -59,6 +60,7 @@ class CobbleSurvival {
         NeoForge.EVENT_BUS.addListener(Zones::onAddReloadListeners)
         Guardians.register()
         ZoneHazards.register()
+        LevelPressure.register()
         SelfTest.register()
     }