feat(phys): add ShadowShape record (no callers yet)

Standalone record representing one collision-bearing shape attached to
a logical PhysicsObj. Foundation for the per-part BSP collision fix
that closes the M1.5 "doors don't block" bug. Spec at
docs/superpowers/specs/2026-05-24-door-collision-per-part-bsp-design.md.

No callers in this commit; integration follows in subsequent commits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/ShadowShape.cs

@@ -0,0 +1,25 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// One collision-bearing shape attached to a logical PhysicsObj.
+/// A door emits 3-4 (CylSphere(s) + Sphere(s) + Part-BSPs); a creature
+/// may emit a few; a simple item zero. Positions and rotations are
+/// LOCAL to the entity's origin so <see cref="ShadowObjectRegistry.UpdatePosition"/>
+/// can re-transform them when the entity moves.
+///
+/// <para>
+/// Retail anchor: <c>CPhysicsPart</c> + <c>CPhysicsPart::find_obj_collisions</c>
+/// at <c>acclient_2013_pseudo_c.txt:275045-275055</c>. Each part stores its
+/// own transform and dispatches per-part collision to its GfxObj.
+/// </para>
+/// </summary>
+public readonly record struct ShadowShape(
+    uint                GfxObjId,
+    Vector3             LocalPosition,
+    Quaternion          LocalRotation,
+    float               Scale,
+    ShadowCollisionType CollisionType,
+    float               Radius,
+    float               CylHeight);