fix(G.3): collapse streaming to the single dungeon landblock indoors (#133 FPS)

Dungeon FPS sat at ~30 (frame ~33ms) because the 25x25 streaming window around
the dungeon landblock pulled in ~129 NEIGHBORING landblocks + their thousands of
torch/particle emitters, all drawn though never visible. In AC all dungeons are
packed adjacent in the unused "ocean" map grid, so those neighbors are unrelated
dungeons. The FPS timeline proved it: 247 fps at login (lb 0/0, ~10K entities) →
17 → 30 as landblocks streamed in (lb 0→129) — the cost tracked LANDBLOCK count,
not entities.

Retail-faithful: ACE LandblockManager.GetAdjacentIDs returns ZERO adjacents for a
dungeon (`if (landblock.IsDungeon) return adjacents;`, Landblock.cs:577-582) —
every dungeon is a self-contained landblock you never see out of.

Fix: when the player stands in a sealed indoor cell (CurrCell.IsEnv &&
!SeenOutside — the same predicate that kills the sun/sky), collapse streaming to
just the player's dungeon landblock and unload the neighbors. Building interiors
(cottage/inn) have SeenOutside cells, so they are NOT gated and keep their
surrounding terrain (the frozen building/cellar demo is unaffected). Unloading the
neighbors also tears down their lights (removeTerrain → UnregisterOwner), shrinking
LightManager._all from ~2227 toward retail's ≤40 — which directly helps the A7
lighting bake landing next.

Mechanics (StreamingController):
- Edge IN: ClearPendingLoads() cancels the in-flight 25x25 window (new streamer
  ClearLoads control job — worker drops queued Loads, keeps Unloads), unload every
  resident neighbor, pin a radius-0 StreamingRegion, (re)load the dungeon block if
  needed.
- Stay collapsed: sweep any straggler that finished loading after the edge (a Load
  the worker had already dequeued before ClearLoads).
- Edge OUT (portal/teleport to outdoors): rebuild the full two-tier window at the
  new center, unload anything stale.

AP-36 added to the divergence register (the gate uses the cheap SeenOutside cell
predicate as an approximation of ACE's full landblock IsDungeon classification).
GameWindow also carries a TEMP ACDREAM_LOG_FPS=1 headless FPS line (strip after
the A7 FPS+lighting verification).

Build green; 58 streaming tests green (6 new dungeon-gate tests).

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

src/AcDream.App/Streaming/LandblockStreamJob.cs

@@ -14,6 +14,16 @@ public abstract record LandblockStreamJob(uint LandblockId)
 {
     public sealed record Load(uint LandblockId, LandblockStreamJobKind Kind) : LandblockStreamJob(LandblockId);
     public sealed record Unload(uint LandblockId) : LandblockStreamJob(LandblockId);
+
+    /// <summary>
+    /// Control job: drop every queued (not-yet-started) Load from the worker's
+    /// priority queues, keeping Unloads. Posted by
+    /// <see cref="LandblockStreamer.ClearPendingLoads"/> when the player enters a
+    /// dungeon and the in-flight outdoor/neighbor window load must be cancelled
+    /// (#133 FPS — dungeons have no adjacent landblocks). LandblockId is 0 by
+    /// convention; readers pattern-match on the type.
+    /// </summary>
+    public sealed record ClearLoads() : LandblockStreamJob(0);
 }
 
 /// <summary>