fix(world+streaming): trees-in-sky Z, O(1) anim, teleport near-ring + immediate unloads

Three apparatus-confirmed fixes from the world-load/FPS deep-dive (all live-verified).

1. trees-in-sky — scenery ground-Z now samples THIS landblock's OWN heightmap
   (TerrainSurface.SampleZFromHeightmap, lock-step with the physics terrain) instead
   of the global PhysicsEngine.SampleTerrainZ query. At build time the landblock isn't
   registered in physics yet, so that query could only return null OR a STALE
   neighbour's height — the previous location's terrain, still registered after a
   teleport recenter — planting scenery at the old altitude (+250..500m, confirmed via
   the [scenery-z-stale] probe). Own-heightmap is correct in every case; the query is
   removed. (GameWindow.BuildSceneryEntitiesForStreaming)

2. FPS per-hop — TickAnimations recovered each animated entity's server guid via an
   O(N) ReferenceEquals reverse scan over ALL _entitiesByServerGuid (which never
   evicts, so N climbs every teleport — the drops-with-each-hop sink). Replaced with
   ae.Entity.ServerGuid: O(1), exact-equivalent (the dict key IS entity.ServerGuid).
   (GameWindow.TickAnimations)

3. teleport arrival + bulk floating terrain — two streaming fixes:
   - Near-ring eager-apply: a teleport applies the destination's 3x3 surroundings
     (StreamingController.PriorityRadius) and holds the fade until they're resident
     (PhysicsEngine.IsNeighborhoodTerrainResident), so the player arrives in a loaded,
     collidable world instead of one landblock in the void.
   - Immediate unloads: DrainAndApply no longer throttles UNLOADS at the per-frame
     load budget — they're cheap (free GPU buffers, no upload). A teleport produced
     ~600 unloads draining at 4/frame, leaving the previous region resident for
     seconds (floating terrain) and accumulating across rapid hops (951 resident vs a
     625 window). Only GPU-upload LOADS are metered now. Cut out-of-window resident
     650 -> 63 and resident 951 -> 688 (live-verified via [resid-audit]).

Includes gated-off diagnostic probes (ACDREAM_PROBE_SCENERY_FRAME / _BLDG_REACH /
_STREAM_RESID) used to root-cause the above — zero-cost when unset, same pattern as
the committed tp-probe.

The pre-existing teleport-induced "terrain arcs in the sky" (present in the dd2eb8b
baseline too, with NONE of this work) are a SEPARATE bug — investigated next.

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -45,6 +45,34 @@ public sealed class PhysicsEngine
         return false;
     }
 
+    /// <summary>
+    /// True once EVERY in-bounds landblock within Chebyshev <paramref name="radius"/> of the
+    /// landblock covering <paramref name="cellOrLandblockId"/> has had its terrain registered.
+    /// This is the teleport "surroundings are loaded" gate: holding the fade until the player's
+    /// own landblock AND its immediate neighbours are resident means they arrive standing on a
+    /// loaded, collidable world (their cell-walk can root into neighbour cells) instead of a
+    /// single landblock floating in the void. Off-map neighbours (coords outside 0..254) are
+    /// skipped — they never load, so requiring them would hang the hold until the wall-clock
+    /// timeout. radius 0 is equivalent to <see cref="IsLandblockTerrainResident"/>.
+    /// </summary>
+    public bool IsNeighborhoodTerrainResident(uint cellOrLandblockId, int radius)
+    {
+        var resident = new HashSet<uint>();
+        foreach (var key in _landblocks.Keys) resident.Add(key & 0xFFFF0000u);
+
+        int cx = (int)((cellOrLandblockId >> 24) & 0xFFu);
+        int cy = (int)((cellOrLandblockId >> 16) & 0xFFu);
+        for (int dx = -radius; dx <= radius; dx++)
+        for (int dy = -radius; dy <= radius; dy++)
+        {
+            int nx = cx + dx, ny = cy + dy;
+            if (nx < 0 || nx > 254 || ny < 0 || ny > 254) continue;   // off-map: skip
+            uint prefix = ((uint)nx << 24) | ((uint)ny << 16);
+            if (!resident.Contains(prefix)) return false;
+        }
+        return true;
+    }
+
     /// <summary>
     /// Cell-based spatial index for static object collision.
     /// Populated during landblock streaming; queried by the Transition system.