Phase 7: one End boss at a time; more visible attack telegraphs

Adrian (2026-11-02): a second Eternatus appeared on his second visit
to the End; the attacks should be more visible.

- EndBoss waits until the entities around its spot are loaded before
  deciding it's missing, refuses a second copy while the recorded one
  is alive (join check, like the guardians), and removes extra copies
  near the spot (cleans up worlds that already have two).
- Telegraphs: 2.2-size dust redrawn every 2 ticks, the inside filled
  (Area.fill), rising sparks on the outline, a low bell on cast.
- Tests: no second End boss (after the End's middle is loaded), fill
  points inside their areas.

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

docs/design.md

@@ -282,13 +282,13 @@ The others fit and stay: Morpeko (Button Isle), Pinsir (Southwold), Scyther (Wes
 
 **Bosses are data**, `datapacks/core/data/cobblesurvival/bosses/<id>.json`, written by `tools/zone_bosses.py` from a tier table per zone band: which fight it is (a zone's guardian, or the End boss), the arena radius, and the attack patterns. A pattern has a shape (circle, ring, line, cone) and its size, where it's aimed (at the player, or around the boss), a telegraph time, damage, an optional effect (fire: set alight; ice: slowed and freezing) and a trigger (every so many seconds, every so many turns, or once when the boss's HP falls below a share).
 
-**Runtime** (companion mod). A battle between players and a boss Pokémon becomes a boss fight. While it lasts, due patterns telegraph with particles on the ground where they'll land, then hit the battling players standing inside; everything stops when the battle ends. The arena is a circle around where the boss stood when the fight began: a player who leaves it, or dies, loses the fight (the battle ends, the boss stays, nothing drops). Cobblemon's own flee check only applies when a battle starts, which is why the arena is enforced here.
+**Runtime** (companion mod). A battle between players and a boss Pokémon becomes a boss fight. While it lasts, due patterns telegraph on the ground where they'll land (made more visible after Adrian's first fight, 2026-11-02: large particles redrawn every 2 ticks over the whole area, a bright outline with rising sparks, and a low bell when an attack begins), then hit the battling players standing inside; everything stops when the battle ends. The arena is a circle around where the boss stood when the fight began: a player who leaves it, or dies, loses the fight (the battle ends, the boss stays, nothing drops). Cobblemon's own flee check only applies when a battle starts, which is why the arena is enforced here.
 
 **Moltres** becomes the Basalt Furnace's guardian like the others (Lv 75, band top + 5), with placeholder drops (3 Exp. Candy XL, 2 netherite scrap).
 
 Self-tests (`BossTests`): every guardian has a fight and a Cobblemon model; each attack shape hits a player standing in it and misses one ten blocks away; attacks stop when the battle ends; leaving the arena ends the battle and the boss heals; the dragon can't join the End, Eternatus appears, and beating it opens the exit portal with the egg.
 
-**Eternatus** replaces the Ender Dragon: the dragon no longer spawns; Eternatus (uncatchable, Lv 80, scaled down to fit) appears on the main End island while a player is there, and beating it does what killing the dragon did (exit portal, gateways, the egg on the first win).
+**Eternatus** replaces the Ender Dragon: the dragon no longer spawns; Eternatus (uncatchable, Lv 80, scaled down to fit) appears on the main End island while a player is there, and beating it does what killing the dragon did (exit portal, gateways, the egg on the first win). Only one at a time: it waits until the entities around its spot have loaded before deciding it's missing, and a second copy never joins while the recorded one is alive (Adrian met a second Eternatus on his second visit, 2026-11-02); extra copies left in older worlds are removed.
 
 ## Open decisions for Adrian (proceeding with the default, flagged)