fix(render): the near plane is col3, not col4 + col3 (#248)

`FrustumPlanes.FromViewProjection` extracted the near plane with the
Gribb-Hartmann form written for OpenGL's `[-1,1]` clip-space z range. Every
acdream projection comes from `Matrix4x4.CreatePerspectiveFieldOfView` or
`CreateOrthographic`, whose range is `[0,1]`. Under `[-1,1]` the near plane is
the locus of `clip.z = -clip.w`, which is `col4 + col3`; under `[0,1]` it is
`clip.z = 0`, which is `col3` alone.

Concretely, the mismatch put the effective near threshold at `-n·f/(2f-n)` —
about 0.5 m where the retail chase camera asks for 1.0 m. That error only ever
kept geometry the true frustum would have dropped, never the reverse, which is
why it produced no visible defect and was filed instead of hot-fixed during
Campaign V. It is still wrong, and it is the same mistake that *was* visible in
`PortalProjection`, where it culled the cell behind a doorway the camera stood
close to.

The far plane is `col4 - col3` under both conventions and is untouched. A test
pins it anyway, so that a future edit to this function cannot drift it while
nobody is looking.

The acceptance criterion asked for a unit test pinning the extracted near
distance to the camera's near value, and that is what landed: a theory over four
near/far pairs asserting the plane is unit-length, faces down -Z, and stands off
the eye by exactly `nearDistance`, plus a kept/dropped pair straddling it. The
test was checked against the old formula before commit and fails all four cases
there — it measures the fix rather than merely accompanying it.

The other half of the acceptance criterion — unchanged culling in the offline
pixel gate and the connected route — could not be run: #259 has Win32 surface
creation failing machine-wide, so no gate that needs a window is available
tonight. Recorded as outstanding rather than assumed.

Solution build 0 errors; `AcDream.Core.Tests` 3,898 passed / 2 skipped / 3,900.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/FrustumCuller.cs

@@ -48,7 +48,18 @@ public readonly struct FrustumPlanes
         var right  = Normalize(col4 - col1);
         var bottom = Normalize(col4 + col2);
         var top    = Normalize(col4 - col2);
-        var near   = Normalize(col4 + col3);
+        // NEAR is the one plane whose formula depends on the clip-space z range.
+        // The familiar `col4 + col3` is the OpenGL form, for NDC z in [-1,1],
+        // where the near plane is the locus of clip.z = -clip.w. Every acdream
+        // projection comes from Matrix4x4.CreatePerspectiveFieldOfView (and
+        // CreateOrthographic), whose NDC z range is [0,1] — there the near plane
+        // is clip.z = 0, which is `col3` by itself. Using the GL form here put
+        // the effective threshold at -n·f/(2f-n), about half the true near
+        // distance, which only ever kept geometry the real frustum would drop.
+        // See docs/ISSUES.md #248; same class as the PortalProjection near-test
+        // bug at PortalProjection.cs:12-19. FAR is `col4 - col3` under BOTH
+        // conventions and is deliberately left alone.
+        var near   = Normalize(col3);
         var far    = Normalize(col4 - col3);
 
         return new FrustumPlanes(left, right, bottom, top, near, far);