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.App/Input/PlayerModeAutoEntry.cs

@@ -8,12 +8,13 @@ namespace AcDream.App.Input;
 /// <c>GameWindow</c> and ticked each frame from <c>OnUpdate</c>.
 ///
 /// <para>
-/// Why is this its own class? The auto-entry has three independent
+/// Why is this its own class? The auto-entry has four independent
 /// preconditions (live session reaches <c>InWorld</c>, the player
-/// entity has been streamed into the world dictionary, and the player
-/// movement controller is constructible) plus a manual-override path
+/// entity has been streamed into the world dictionary, the player
+/// movement controller is constructible, and the terrain under the
+/// spawn position has streamed in) plus a manual-override path
 /// (the user can flip into fly mode before the auto-entry fires —
-/// their choice wins). All four interact with each other in a way
+/// their choice wins). All five interact with each other in a way
 /// that's painful to test through GameWindow but trivial here against
 /// fakes.
 /// </para>
@@ -43,6 +44,7 @@ public sealed class PlayerModeAutoEntry
     private readonly Func<bool> _isLiveInWorld;
     private readonly Func<bool> _isPlayerEntityPresent;
     private readonly Func<bool> _isPlayerControllerReady;
+    private readonly Func<bool> _isSpawnGroundReady;
     private readonly Action     _enterPlayerMode;
 
     private bool _armed;
@@ -60,6 +62,12 @@ public sealed class PlayerModeAutoEntry
     /// PlayerMovementController is set up. Stays true once player mode
     /// is established; the auto-entry's job is to flip it from false
     /// to true exactly once.</param>
+    /// <param name="isSpawnGroundReady">True iff the terrain under the
+    /// player's spawn position has streamed into the physics engine.
+    /// #106 gate-2 (2026-06-09): entering player mode earlier integrates
+    /// gravity against an empty world and free-falls the player into the
+    /// void. Retail never has this state — it loads cells synchronously;
+    /// this hold is the async-streaming equivalent of that invariant.</param>
     /// <param name="enterPlayerMode">Action invoked on the firing
     /// tick. The same routine the manual Tab handler invokes (fly →
     /// player transition). Must construct the controller + chase
@@ -69,11 +77,13 @@ public sealed class PlayerModeAutoEntry
         Func<bool> isLiveInWorld,
         Func<bool> isPlayerEntityPresent,
         Func<bool> isPlayerControllerReady,
+        Func<bool> isSpawnGroundReady,
         Action     enterPlayerMode)
     {
         _isLiveInWorld           = isLiveInWorld           ?? throw new ArgumentNullException(nameof(isLiveInWorld));
         _isPlayerEntityPresent   = isPlayerEntityPresent   ?? throw new ArgumentNullException(nameof(isPlayerEntityPresent));
         _isPlayerControllerReady = isPlayerControllerReady ?? throw new ArgumentNullException(nameof(isPlayerControllerReady));
+        _isSpawnGroundReady      = isSpawnGroundReady      ?? throw new ArgumentNullException(nameof(isSpawnGroundReady));
         _enterPlayerMode         = enterPlayerMode         ?? throw new ArgumentNullException(nameof(enterPlayerMode));
     }
 
@@ -107,6 +117,7 @@ public sealed class PlayerModeAutoEntry
         if (!_isLiveInWorld()) return false;
         if (!_isPlayerEntityPresent()) return false;
         if (!_isPlayerControllerReady()) return false;
+        if (!_isSpawnGroundReady()) return false;
 
         _armed = false;
         _enterPlayerMode();