fix(phys): #106 gate-2 — bogus-indoor-claim recovery + spawn-ground entry hold

Gate-2 fallout chain: session 1's bare-id wedge poisoned ACE's save (the
client reported garbage cells while walking through walls), so session 2
logged in with (cell=0xA9B4013F inn interior, pos=(91.4,32.7)) — a
position that cell does not contain. Probe evidence: exactly one
[cell-transit] line all session; the player free-fell into an empty
world. Two holes, both fixed at the root:

1. CellTransit pick escape hatch — restores the #83/A1.7 + #90
   verification that lived in PhysicsEngine.ResolveCellId before the
   collide-then-pick rewrite moved membership into
   BuildCellSetAndPickContaining: an indoor current cell that IS
   hydrated but whose CellBSP no longer overlaps ANY part of the foot
   sphere is a bogus claim (corrupt save, or walked out through an
   unblocked gap). The portal BFS can never reach an exit portal from a
   cell the sphere isn't in, so no candidates exist and the claim held
   forever — wedging collision (ShadowObjectRegistry's #98 gate reads
   "indoor primary" -> outdoor object sweep skipped), wall BSP, terrain,
   and the render root. The pick now demotes to the outdoor column under
   the sphere centre (the LandDefs.AdjustToOutside result already
   computed for the pick — cross-block safe). Sphere-overlap
   (BSPQuery.SphereIntersectsCellBsp, pseudo_c:317666 -> :323267), NOT
   point-in: doorway push-back leaves the centre a few cm outside while
   the sphere still overlaps — no demotion, #90's ping-pong stays dead.
   An unhydrated cell cannot be verified — stale beats null while
   streaming hydrates (retail-equivalent: stale curr_cell kept when the
   pick finds nothing).

2. PlayerModeAutoEntry spawn-ground hold — player-mode entry now waits
   for the terrain under the spawn position to stream in
   (isSpawnGroundReady predicate, K.2 pattern). Entering earlier
   integrates gravity against an empty world: indoor-claimed spawns got
   no floor from any source and free-fell into the void; outdoor spawns
   raced hydration by ~1s every login. Retail never has this state (it
   loads cells synchronously) — the hold is the async-streaming
   equivalent of that invariant. With the hold, the entry snap
   (Resolve, stepUp=100) runs against hydrated cell floors + terrain
   and re-seats a corrupt save's claim immediately.

Tests: IndoorSeed_SphereFullyOutsideHydratedCell_DemotesToOutdoorColumn
(the gate-2 wedge shape, red pre-fix), straddle + no-BSP guards (the #90
hysteresis and stale-beats-null), TryEnter_Armed_SpawnGroundNotReady_
DoesNotFire. Full suite: 294+218+420 green; Core 1375 green + the same
4 pre-existing door/#99-era failures + 1 skip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Core/Physics/CellTransit.cs

@@ -619,7 +619,40 @@ public static class CellTransit
         // No interior cell contained the centre. Return the outdoor XY-column cell if
         // it was a candidate, else stay on the current cell (retail leaves *result
         // null → caller keeps curr_cell).
-        return outdoorResult != 0u ? outdoorResult : currentCellId;
+        if (outdoorResult != 0u) return outdoorResult;
+
+        // ── #106 gate-2 escape hatch: bogus-indoor-claim recovery ──────────
+        // Restores the #83/A1.7 + #90 verification that lived in
+        // PhysicsEngine.ResolveCellId before the collide-then-pick rewrite
+        // moved membership here: an INDOOR current cell that IS hydrated but
+        // whose CellBSP no longer overlaps ANY part of the foot sphere is a
+        // bogus claim — a corrupt server save pairing an indoor cell with a
+        // position far outside it, or the player walked out through an
+        // unblocked gap. Keeping it wedges everything downstream: the BFS
+        // can't reach an exit portal from a cell the sphere isn't in (no
+        // candidates → frozen), ShadowObjectRegistry's #98 gate reads
+        // "indoor primary" (no object collision anywhere), and there's no
+        // wall BSP and no terrain (void fall). Demote to the outdoor column
+        // under the sphere centre (LandDefs global math — cross-block safe).
+        //
+        // Sphere-overlap (BSPQuery.SphereIntersectsCellBsp,
+        // pseudo_c:317666→:323267), NOT point-in: a doorway push-back leaves
+        // the centre a few cm outside while the sphere still overlaps — that
+        // must NOT demote (#90's ping-pong). A cell with no hydrated CellBSP
+        // cannot be verified — trust the claim (stale beats null while
+        // streaming hydrates).
+        if (currentLow >= 0x0100u && containingOutdoorId != 0u)
+        {
+            var cur = cache.GetCellStruct(currentCellId);
+            if (cur?.CellBSP?.Root is not null)
+            {
+                var curLocal = Vector3.Transform(worldSphereCenter, cur.InverseWorldTransform);
+                if (!BSPQuery.SphereIntersectsCellBsp(cur.CellBSP.Root, curLocal, sphereRadius))
+                    return containingOutdoorId;
+            }
+        }
+
+        return currentCellId;
     }
 
     private static int EffectiveSphereCount(IReadOnlyList<Sphere> worldSpheres, int numSpheres)