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>

docs/DECISIONS.md

@@ -403,6 +403,29 @@ asserts the burst lands where the marker promised.
 other thing in the view. A warning you cannot see is an unavoidable hit with
 extra steps, which is the opposite of what a telegraph is for.
 
-**Which boss a run has comes from its seed, not its 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.
+**Each boss has its own entrance in the hub.** A dungeon names the arena it ends
+in, and the arena decides the boss. Which fight you are walking into is the one
+thing a player chooses before a run, so it is not rolled for them — not from the
+depth (a dev flag nothing in play raises, which left the second boss
+unreachable) and not from the seed either. Only the Proving Grounds leaves it
+open, because a harness you re-enter every couple of minutes wants whichever
+fight comes up first.
+
+---
+
+## Bullets do not animate
+
+**One frame, and a slow turn.** The bullet sheet's eight frames are a colour
+cycle rather than a shape change, and running it meant every bullet on screen
+strobed through a palette in unison — genuinely hard to look at with a few
+hundred in the air, which is the normal case for this game.
+
+Each bullet now holds one frame and rotates at `Art.BULLET_SPIN_RATE`, offset by
+its own id so a ring of twenty does not turn 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 as spin — a test pins that against the
+longest-lived bullet in the game.
+
+The other seven frames stay in the atlas because that is what the source art is.
+Nothing draws them, and the renderer now slices four textures at startup instead
+of thirty-two.