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>

tests/unit/test_boss_movement.gd

@@ -254,8 +254,8 @@ func test_a_replica_never_moves_a_boss() -> void:
 ## circuit, chases, orbits and telegraphs runs for thousands of ticks without
 ## leaving its room, standing in a wall, or firing nothing.
 func test_the_cantor_survives_its_own_fight() -> void:
-	var inst := Instance.make_dungeon(2, 9183, 1)
-	assert_eq(inst.boss_id, Content.BOSS_CANTOR, "setup: an odd seed is the vault")
+	var inst := Instance.make_dungeon(2, 9183, 1, Dungeons.VAULT)
+	assert_eq(inst.boss_id, Content.BOSS_CANTOR, "setup: the vault is the Cantor's")
 	var b := inst.world.boss
 	var bait := inst.world.add_player(1, "bait")
 	bait.pos = b.pos + Vector2(180.0, 0.0)