feat(core): Phase B.3 — PhysicsEngine (top-level collision resolver)

Combines TerrainSurface + CellSurface into a single Resolve() API
that handles outdoor terrain walking, indoor floor walking,
outdoor<->indoor cell transitions, step-height enforcement, and
ground detection.

Step-height blocks upward Z deltas exceeding the limit (walls,
cliffs); downhill movement is always accepted. Indoor transitions
pick the cell whose floor Z is closest to the entity's current Z
(handles multi-story buildings). Reports IsOnGround=false when
no landblock or surface covers the entity's position (gravity
applied by the caller).

One API mismatch fixed vs plan: plan encoded the upper 16 landblock
bits into the returned cell ID, but the tests assert the raw cell ID
(0x0100, <0x0100) — so Resolve returns targetCellId directly.

6 new tests covering flat terrain, slopes, step-height rejection,
indoor entry/exit, and void detection. 243 total, all green.

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

src/AcDream.Core/Physics/ResolveResult.cs

@@ -0,0 +1,13 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Result of <see cref="PhysicsEngine.Resolve"/>: the validated
+/// position after collision, the cell the entity ended up in,
+/// and whether they're standing on a surface.
+/// </summary>
+public readonly record struct ResolveResult(
+    Vector3 Position,
+    uint CellId,
+    bool IsOnGround);