Phase 3: Alpha guardians, one per zone, back 5 hours after a defeat

- Companion mod (zone/Guardians.kt): each zone's guardian stands at the
  zone's sample point, appears when a player is within 96 blocks, stays
  within 24 blocks, is uncatchable and half again as big. Beating it in a
  battle gives every player in the battle its drops; it returns after 5
  hours of game time (Adrian: fixed spots, 5 hours). State lives in the
  world save; /cobblesurvival guardians lists it.
- tools/zone_spawns.py writes data/cobblesurvival/guardians/<zone>.json:
  the strongest walking (or swimming) Pokemon of the zone at band top + 5,
  with the drops from world/zones.json. Boss drops follow Adrian's rule:
  Icy Rocks before Sunscorch, Heat Rocks before Frostpeak.
- Goat Fur and Hoglin Hide recipes make 8 per rock, so one guardian's 3
  rocks pay for one full insulating set.
- Self-test guardian_appears_and_returns; hazard tests cleaned up; the
  zone generator reads Cobblemon's own entries from the field guide's
  hidden_spawns.

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

README.md

@@ -67,7 +67,7 @@ Lays the zone plan in `world/zones.json` onto the island (from the seed's biome
 ```bash
 python3 tools/zone_spawns.py
 ```
-Writes each zone's spawn pool from Cobblemon's spawn data (every evolution line on the island gets one home zone; levels follow the zone's band) and lists the result in `docs/zone-species.md`. Run after `tools/zone_layout.py` and the field guide; pin a line to a zone in `world/zones.json` → `species`.
+Writes each zone's spawn pool from Cobblemon's spawn data (every evolution line on the island gets one home zone; levels follow the zone's band) and its Alpha guardian, and lists both in `docs/zone-species.md`. Run after `tools/zone_layout.py` and the field guide; pin a line to a zone in `world/zones.json` → `species`.
 
 ```bash
 python3 tools/fieldguide/build.py