feat(physics): Task 2 — true sphere collision primitive (CSphere::intersects_sphere)

Setup.Spheres were previously coerced to short cylinders (CylHeight=2*r),
which is geometrically wrong: a cylinder has flat caps; a sphere does not.
This ported CSphere::intersects_sphere (0x00537A80) so sphere-typed shadow
entries are tested as spheres — 3-D distance, no height clamping.

Changes:
- ShadowObjectRegistry.cs: added ShadowCollisionType.Sphere (enum value 2).
  The BuildFloodSpheres anyCyl dedup at :232 is unaffected: only Cylinder
  sets anyCyl=true; Sphere shapes fall through to the BSP-fallback path
  (anyCyl=false → included), which is correct.
- ShadowShapeBuilder.cs: FromSetup now emits ShadowCollisionType.Sphere
  (CylHeight=0) for Setup.Spheres instead of a short Cylinder.
- CollisionPrimitives.cs: added SweptSphereHitsSphere — quadratic swept
  solve ported from ACE Sphere.cs::FindTimeOfCollision, which is a C# port
  of retail's CSphere::intersects_sphere @ 0x00537A80. Sign convention
  confirmed against the decomp: retail negates the root to produce a
  forward t ∈ (0,1].
- TransitionTypes.cs: added Sphere narrow-phase branch between BSP and
  Cylinder in FindObjCollisionsInCell; uses 3-D distance for overlap
  (not XY-only). Added SphereCollision() method implementing the 3-D
  wall-slide response. Updated diagnostic logging at :2734 to cover Sphere.
- Updated ShadowShapeBuilderTests for new Sphere type assertion.
- New SphereIntersectsSphereConformanceTests: 9 geometrically-anchored
  cases (head-on, tangent, perpendicular-miss, lateral-near-miss,
  sweep-away, beyond-step, degenerate-zero-sweep, already-overlapping,
  vertical-sweep).

Retail oracle: CSphere::intersects_sphere @ 0x00537A80 (named-retail);
ACE Sphere.cs::FindTimeOfCollision (C# port, cross-confirmed).
Build: 0 errors, 10 warnings (pre-existing).
Tests: 1576 pass / 0 fail / 2 skip (1578 total).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/ShadowShapeBuilder.cs

@@ -64,7 +64,9 @@ public static class ShadowShapeBuilder
         }
 
         // 2. Spheres — only when no CylSpheres (matches landblock-static convention
-        //    at GameWindow.cs:6034). Each becomes a short Cylinder.
+        //    at GameWindow.cs:6034). Each becomes a true Sphere (no height clamping).
+        //    Retail anchor: CSphere::intersects_sphere @ 0x00537A80 uses 3-D distance
+        //    for the overlap check, unlike CCylSphere which clips to [low_pt, high_pt].
         if (setup.CylSpheres.Count == 0)
         {
             foreach (var sph in setup.Spheres)
@@ -75,9 +77,9 @@ public static class ShadowShapeBuilder
                     LocalPosition: new Vector3(sph.Origin.X, sph.Origin.Y, sph.Origin.Z) * entScale,
                     LocalRotation: Quaternion.Identity,
                     Scale:         entScale,
-                    CollisionType: ShadowCollisionType.Cylinder,
+                    CollisionType: ShadowCollisionType.Sphere,
                     Radius:        sph.Radius * entScale,
-                    CylHeight:     sph.Radius * 2f * entScale));
+                    CylHeight:     0f));
             }
         }