feat(teleport C): TAS-driven fade transit + retire TeleportArrivalController

Wires the dormant TeleportAnimSequencer as the transit driver: on PlayerTeleport
the player holds in PortalSpace behind a full-screen fade (FadeOverlay) until the
destination terrain is resident (TeleportWorldReady, gated on the priority-applied
landblock), then materializes (Place), and after the world fades back in regains
control + acks the server (FireLoginComplete). No movement resolves against the
empty world, so the outbound cell frame can't corrupt. Outdoor changes from
place-immediately back to hold-until-resident (now fast, not a band-aid).

- FadeOverlay: fullscreen NDC black quad, alpha = ShowTunnel ? 1 : FadeAlpha.
- Retires TeleportArrivalController + its 2 tests (TAS subsumes the driver role).
- Divergence register: AD-2 updated to the new mechanism; AD-31 (fade vs swirl).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Streaming/DungeonStreamingGate.cs

@@ -37,8 +37,8 @@ public static class DungeonStreamingGate
         // must follow the DESTINATION, which the PortalSpace observer pin already does, so
         // the source-cell gate is suppressed. Otherwise a teleport OUT of a dungeon keeps
         // streaming collapsed on the source dungeon (CurrCell still sealed) → the outdoor
-        // destination never hydrates → TeleportArrivalReadiness holds 600 frames → force-
-        // snap to ocean. A teleport INTO a dungeon is handled explicitly upstream by
+        // destination never hydrates → the TAS transit holds 600 frames → force-snap to
+        // ocean. A teleport INTO a dungeon is handled explicitly upstream by
         // StreamingController.PreCollapseToDungeon (and the controller's _collapsed latch
         // holds it through the hold), so suppressing the gate here doesn't regress it.
         if (isTeleportHold)