The Cantor gets its own portal; bullets stop strobing

A dungeon now names the boss arena it ends in, and the arena decides the boss.
Three entrances in the hub: Warden's Descent, The Choir Vault, Proving Grounds.

That replaces the seed coin-flip from the last commit, which was the wrong
call. Which boss you are about to fight is the single thing a player decides
before walking into a dungeon, and rolling it for them makes that decision
unavailable. Only the Proving Grounds still leaves its arena open, because a
harness you re-enter every couple of minutes wants whichever fight comes up
first -- and it is the one place where "either will do" is true.

test_every_boss_has_a_dungeon_that_reaches_it is the check that matters here:
adding a boss and forgetting to give it a way in is now a failing test rather
than a boss nobody meets. The smoke test asserts all three portals open over a
real socket -- bots pick theirs by account id, so a run exercises each.

Bullets no longer animate. The sheet's eight frames are a COLOUR cycle rather
than a shape change, so running it had every bullet on screen strobing through
a palette in unison, which with a few hundred in the air is exactly as hard to
look at as it sounds. Each bullet holds one frame and turns slowly instead,
offset by its own id so a ring of twenty does not rotate as one rigid wheel.
The rate is slow enough that nothing completes a full turn inside its own
lifetime, so it reads as drift rather than spin, and a test pins that against
the longest-lived bullet in the game. The renderer slices four textures at
startup now instead of thirty-two.

check.sh clean, 414 tests, SMOKE PASS (22 assertions), all four diagnostics
green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/instances/instance.gd

@@ -66,7 +66,9 @@ static func make_dungeon(instance_id: int, dungeon_seed: int, dungeon_depth: int
 	inst.depth = maxi(dungeon_depth, 1)
 	inst.dungeon_id = dungeon if not dungeon.is_empty() else Dungeons.default_id()
 	inst.world = SimWorld.new(dungeon_seed)
-	var built := MapGen.build(Protocol.InstanceKind.DUNGEON, dungeon_seed, inst.depth)
+	var flavour := Dungeons.get_or_default(inst.dungeon_id)
+	var built := MapGen.build(Protocol.InstanceKind.DUNGEON, dungeon_seed,
+		inst.depth, flavour.arena)
 	inst.world.set_map(built["grid"])
 	inst.rooms = built["rooms"]
 	inst.boss_spawn = built["boss_pos"]