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/Input/AutoEnterPlayerModeTests.cs

@@ -24,6 +24,9 @@ public sealed class AutoEnterPlayerModeTests
         public bool LiveInWorld;
         public bool PlayerEntityPresent;
         public bool PlayerControllerReady;
+        // Defaults TRUE: the hydration hold is not under test in the
+        // original K.2 cases (see SpawnGroundNotReady test for it).
+        public bool SpawnGroundReady = true;
         public int  EnteredCount;
 
         public PlayerModeAutoEntry Build() =>
@@ -31,9 +34,37 @@ public sealed class AutoEnterPlayerModeTests
                 isLiveInWorld:           () => LiveInWorld,
                 isPlayerEntityPresent:   () => PlayerEntityPresent,
                 isPlayerControllerReady: () => PlayerControllerReady,
+                isSpawnGroundReady:      () => SpawnGroundReady,
                 enterPlayerMode:         () => EnteredCount++);
     }
 
+    [Fact]
+    public void TryEnter_Armed_SpawnGroundNotReady_DoesNotFire()
+    {
+        // #106 gate-2 (2026-06-09): player physics must not start before
+        // the terrain under the spawn position has streamed in — entering
+        // earlier free-falls the player into the void (gravity integrates
+        // against an empty world; retail never has this state because it
+        // loads cells synchronously). The guard holds until the streaming
+        // pipeline registers the spawn landblock.
+        var s = new State
+        {
+            LiveInWorld = true, PlayerEntityPresent = true,
+            PlayerControllerReady = true, SpawnGroundReady = false,
+        };
+        var guard = s.Build();
+        guard.Arm();
+
+        Assert.False(guard.TryEnter());
+        Assert.Equal(0, s.EnteredCount);
+        Assert.True(guard.IsArmed);
+
+        // Terrain hydrates → fires on the next tick.
+        s.SpawnGroundReady = true;
+        Assert.True(guard.TryEnter());
+        Assert.Equal(1, s.EnteredCount);
+    }
+
     [Fact]
     public void TryEnter_NotArmed_DoesNotFire()
     {