fix(app): Phase B.2 — use server position directly, fix yaw wrap + turn spam

Three more fixes from the diagnostic dump:

1. Initial position: PhysicsEngine.Resolve was mapping the player
   into an indoor EnvCell (foundry at Z=66) when they're standing
   on outdoor terrain at Z=93+. The cell-containment check was too
   aggressive for initial placement. Now uses the server-sent
   position directly — the server already gave us a valid position.

2. Yaw unbounded: mouse delta accumulated without wrapping, growing
   to 24+ radians. Now wraps to [-PI, PI] after every turn.

3. Turn command spam: MouseDeltaX > 0.5 threshold was too low for
   raw pixel deltas. Any mouse jitter triggered turnCmd flips every
   frame → stateChanged=True → MoveToState flood to the server.
   Mouse turning now only affects yaw directly; turn COMMANDS only
   come from A/D keyboard (matching retail client behavior where
   mouse-look doesn't generate a TurnRight/TurnLeft command).

265 tests still green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -21,6 +21,9 @@ public sealed class PhysicsEngine
 {
     private readonly Dictionary<uint, LandblockPhysics> _landblocks = new();
 
+    /// <summary>Number of registered landblocks (diagnostic).</summary>
+    public int LandblockCount => _landblocks.Count;
+
     private sealed record LandblockPhysics(
         TerrainSurface Terrain,
         IReadOnlyList<CellSurface> Cells,