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/checklists/phase-7.md

@@ -18,6 +18,7 @@ On the island save (or a new one), in creative or survival:
 - [ ] Fight the Button Isle guardian (Morpeko, `/cobblesurvival guardians` lists where they stand): red circles appear under you every few seconds; step out before they land. Close the battle screen to move (Cobblemon's own key), then go back to it.
 - [ ] Fight a later guardian (e.g. Flygon in Sunscorch or Swampert in the Mire): lines and cones from the boss as well.
 - [ ] Walk more than 24 blocks away from a guardian during its fight: the battle ends as lost, and the guardian is at full HP again.
+- [ ] Leave the End and come back without beating Eternatus: still only one (fixed 2026-11-02).
 - [ ] Go to the End: no dragon; Eternatus stands south of the portal, the boss bar shows its HP. Beat it: the exit portal opens, the egg appears.
 - [ ] Find Regigigas in a Deep Dark cave and Moltres in the Basalt Furnace.