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>

tests/AcDream.Core.Tests/Physics/CellTransitFindCellSetTests.cs

@@ -228,6 +228,103 @@ public class CellTransitFindCellSetTests
         Assert.Equal(0xA9B40031u, containing);
     }
 
+    // ──────────────────────────────────────────────────────────────────
+    // #106 gate-2 escape hatch — bogus indoor claim recovery.
+    // Restores the #83/A1.7 + #90 verification (formerly in ResolveCellId,
+    // lost in the collide-then-pick rewrite): a HYDRATED indoor current
+    // cell whose CellBSP no longer overlaps ANY part of the foot sphere is
+    // a bogus claim (corrupt server save pairing an indoor cell with a
+    // position far outside it, or walked out through an unblocked gap).
+    // Staying on it wedges everything: the BFS can't reach an exit portal
+    // from a cell the sphere isn't in (no outdoor candidates, membership
+    // frozen), GetNearbyObjects' #98 gate reads "indoor primary" (no
+    // object collision), no wall BSP and no terrain (void fall). The pick
+    // must demote to the outdoor column under the sphere centre.
+    // Sphere-overlap (not point-in) preserves the #90 doorway-pushback
+    // hysteresis.
+    // ──────────────────────────────────────────────────────────────────
+
+    /// <summary>Cell whose CellBSP half-space is x ≥ 0 (cell-local).</summary>
+    private static CellPhysics MakeCellWithBoundedBsp(Matrix4x4 worldTransform)
+    {
+        Matrix4x4.Invert(worldTransform, out var inv);
+        return new CellPhysics
+        {
+            WorldTransform        = worldTransform,
+            InverseWorldTransform = inv,
+            Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
+            CellBSP = new CellBSPTree
+            {
+                Root = new CellBSPNode
+                {
+                    SplittingPlane = new Plane(new Vector3(1f, 0f, 0f), 0f),
+                    PosNode        = new CellBSPNode { Type = BSPNodeType.Leaf },
+                },
+            }
+        };
+    }
+
+    [Fact]
+    public void IndoorSeed_SphereFullyOutsideHydratedCell_DemotesToOutdoorColumn()
+    {
+        // The gate-2 live wedge shape: claimed cell 0xA9B40150, sphere far
+        // outside its volume (x = -10, fully behind the x≥0 half-space).
+        // The BFS finds no portals (sphere nowhere near them), so no outdoor
+        // candidates exist — pre-fix this returned the bogus claim forever.
+        // Expected: the global outdoor column under the centre. x=-10 is one
+        // cell WEST of the A9B4 block edge → block 0xA8B4, lcoord (1351,1440)
+        // → cell (7,0) → low 0x39.
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40150u, MakeCellWithBoundedBsp(Matrix4x4.Identity));
+
+        uint containing = CellTransit.FindCellSet(
+            cache, new Vector3(-10f, 12f, 0f), sphereRadius: 0.5f,
+            currentCellId: 0xA9B40150u,
+            out _);
+
+        Assert.Equal(0xA8B40039u, containing);
+    }
+
+    [Fact]
+    public void IndoorSeed_SphereStraddlesCellBoundary_StaysCurrent()
+    {
+        // #90 hysteresis guard: centre just outside the half-space (x=-0.3)
+        // but the 0.5 radius still reaches back in → sphere OVERLAPS → the
+        // claim is legitimate (doorway push-back shape) → NO demotion.
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40150u, MakeCellWithBoundedBsp(Matrix4x4.Identity));
+
+        uint containing = CellTransit.FindCellSet(
+            cache, new Vector3(-0.3f, 12f, 0f), sphereRadius: 0.5f,
+            currentCellId: 0xA9B40150u,
+            out _);
+
+        Assert.Equal(0xA9B40150u, containing);
+    }
+
+    [Fact]
+    public void IndoorSeed_CellWithoutBsp_CannotVerify_StaysCurrent()
+    {
+        // Stale-beats-null while streaming hydrates: a registered cell with
+        // no CellBSP yet cannot be verified — trust the claim (no demotion).
+        Matrix4x4.Invert(Matrix4x4.Identity, out var inv);
+        var cellNoBsp = new CellPhysics
+        {
+            WorldTransform        = Matrix4x4.Identity,
+            InverseWorldTransform = inv,
+            Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
+        };
+        var cache = new PhysicsDataCache();
+        cache.RegisterCellStructForTest(0xA9B40150u, cellNoBsp);
+
+        uint containing = CellTransit.FindCellSet(
+            cache, new Vector3(-10f, 12f, 0f), sphereRadius: 0.5f,
+            currentCellId: 0xA9B40150u,
+            out _);
+
+        Assert.Equal(0xA9B40150u, containing);
+    }
+
     // ──────────────────────────────────────────────────────────────────
     // Membership hysteresis — the R1-flap root cause.
     // Retail CObjCell::find_cell_list adds the CURRENT cell at index 0