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/sim/map_gen.gd

@@ -19,10 +19,11 @@ extends RefCounted
 ## Generation is still deterministic from (kind, seed, depth), which is what
 ## makes a bad run reproducible from its log line; tests/unit/test_map_gen.gd
 ## pins that.
-static func build(kind: Protocol.InstanceKind, seed_value: int, depth: int) -> Dictionary:
+static func build(kind: Protocol.InstanceKind, seed_value: int, depth: int,
+		arena: StringName = &"") -> Dictionary:
 	if kind == Protocol.InstanceKind.LOBBY:
 		return _build_lobby()
-	return generate(seed_value, depth)
+	return generate(seed_value, depth, arena)
 
 
 static func _build_lobby() -> Dictionary:
@@ -71,7 +72,11 @@ static func _build_lobby() -> Dictionary:
 
 ## Result keys: grid, rooms (Array[Rect2i]), spawn (Vector2), boss_pos
 ## (Vector2), boss_room (Rect2i).
-static func generate(seed_value: int, depth: int) -> Dictionary:
+## [param arena] names the boss arena to stamp, and therefore which boss the run
+## ends with. Empty leaves it to the seed, which is what the Proving Grounds
+## wants and nothing else does -- for a real dungeon the boss is the thing a
+## player chose at the portal.
+static func generate(seed_value: int, depth: int, arena: StringName = &"") -> Dictionary:
 	var rng := RandomNumberGenerator.new()
 	rng.seed = seed_value
 	var d := maxi(depth, 1)
@@ -86,16 +91,17 @@ static func generate(seed_value: int, depth: int) -> Dictionary:
 
 	# The boss arena is placed first and everything else works around it, so a
 	# generated corridor can never carve through the authored fight.
-	# Which arena -- and therefore which boss -- comes from the SEED, not the
-	# depth. Depth is a dev flag that nothing in play raises, so keying the
-	# arena to it meant the second boss existed and no player could ever reach
-	# it. Every run rolls a fresh seed, so this is a coin flip per dungeon.
-	var arena := &"warden_hall" if posmod(seed_value, 2) == 0 else &"choir_vault"
-	var stamp := Rooms.warden_hall() if arena == &"warden_hall" else Rooms.choir_vault()
+	# Normally the dungeon says which arena it is; only the test harness leaves
+	# it open, and then the seed decides.
+	var chosen := arena
+	if chosen.is_empty():
+		chosen = &"warden_hall" if posmod(seed_value, 2) == 0 else &"choir_vault"
+	var stamp := Rooms.warden_hall() if chosen == &"warden_hall" else Rooms.choir_vault()
 	var bs := Rooms.size_of(stamp)
 	var boss_origin := Vector2i(w - bs.x - 2, (h - bs.y) / 2)
 	var markers := Rooms.stamp(grid, stamp, boss_origin)
 	var boss_room := Rect2i(boss_origin, bs)
+	var arena_id := chosen
 
 	var boss_pos := grid.tile_centre(
 		boss_origin.x + bs.x / 2, boss_origin.y + bs.y / 2)
@@ -165,7 +171,7 @@ static func generate(seed_value: int, depth: int) -> Dictionary:
 		# Which boss lives here follows from WHICH arena was stamped. Choosing
 		# it anywhere else would let a dungeon put the Cantor in the Warden's
 		# hall, where its patterns assume geometry that is not there.
-		"boss_id": Rooms.boss_for_arena(arena),
+		"boss_id": Rooms.boss_for_arena(arena_id),
 	}