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>

tools/smoke.sh

@@ -128,7 +128,9 @@ check  "a boss spawned"              "$OUT/server.log" "BOSS_SPAWNED (warden|can
 # opens one of each kind. This is what catches a portal wired to the wrong
 # dungeon, or an instance matcher that ignores which dungeon was asked for.
 check  "the real run opened"         "$OUT/server.log" "opened dungeon instance [0-9]+ \(warden_descent\)"
+check  "the choir vault opened"      "$OUT/server.log" "opened dungeon instance [0-9]+ \(choir_vault\)"
 check  "the proving grounds opened"  "$OUT/server.log" "opened dungeon instance [0-9]+ \(proving_grounds\)"
+check  "and each ended with its own boss" "$OUT/server.log" "BOSS_SPAWNED cantor in instance"
 check  "emergency escape completed"  "$OUT/server.log" "escaped to lobby"
 check  "bot1 reached a dungeon"      "$OUT/bot1.log"   "entered instance .*DUNGEON"
 check  "bot1 returned to the lobby"  "$OUT/bot1.log"   "entered instance .*LOBBY"