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>

docs/checklists/phase-3.md

@@ -21,7 +21,9 @@ Open `build/zone-map.png` (the island coloured by zone) and read the draft in `d
 
 - **The Mire's poison** (step 5). Mire Waders from the Westmarch and Jungle guardians keep it off. Automated: `mire_poisons_without_waders`.
 
-Still to come: soft level pressure, flyers.
+- **Soft level pressure** (step 6). Pokémon far above a zone's band learn less there. Automated: `level_pressure_cuts_xp`.
+
+Still to come: flyers.
 
 ## In-game
 
@@ -33,3 +35,4 @@ Still to come: soft level pressure, flyers.
 - [ ] **Lowlands:** a night in Southwold or on Button Isle without gear never gets dangerous.
 - [ ] **Guardians:** `/cobblesurvival guardians` (operator) lists each zone's guardian and spot. Walk there: a big Pokémon appears, can't be caught, and beating it puts its drops in your inventory with a gold message. It's gone for 5 hours of play.
 - [ ] **Mire:** walk into the Mangrove Mire: a green warning, then Poison keeps coming back. With Mire Waders on (from the Westmarch or Jungle guardian) nothing happens.
+- [ ] **Level pressure:** bring a Lv 30 Pokémon to Button Isle and win a battle: it gains only a sliver of experience, while a Lv 10 one gains the full amount.