fix(N.5b): black terrain — switch to uvec2 handle + sampler constructor

Symptom: terrain renders pure black in modern path (legacy renderer
correct). Diagnostic at TerrainModernRenderer.Draw showed:
  glProgramUniformHandle(prog=4, loc=5, handle=0x100251xxx) → GL_INVALID_OPERATION (0x0502)
on both terrain and alpha sampler uniforms.

Root cause: the `uniform sampler2DArray` + glProgramUniformHandleARB
combination is rejected by the NVIDIA Windows driver in this configuration.
The handle is valid and resident; the uniform location is valid; the
program is valid; but the driver refuses to bind a 64-bit handle to a
sampler uniform via the program-uniform path.

Fix: switch to N.5's mesh_modern pattern — pass each 64-bit handle as a
`uniform uvec2` (low + high 32-bit halves) and construct the sampler at
the use site via the GLSL `sampler2DArray(handle)` constructor. This
form is what ARB_bindless_texture documents as universally supported and
is what N.5 already uses successfully.

Files:
- terrain_modern.frag: replace `uniform sampler2DArray uTerrain/uAlpha`
  with `uniform uvec2 uTerrainHandle/uAlphaHandle` + `#define`s
- TerrainModernRenderer.cs: cache uvec2 uniform locations; set via
  `glProgramUniform2(program, loc, low32, high32)` per frame
- BindlessSupport.cs: remove now-unused `SetSamplerHandleUniform`,
  leave a comment noting why the helper was retired
- GameWindow.cs: also strip the temporary [TERRAIN-DBG] cursor-wrap
  print added during the perf-baseline investigation

Build green; 114/114 tests in N.5+N.5b filter still pass; user-verified
terrain renders correctly in modern path post-fix. Captured fresh perf
baseline:
- Legacy:  cpu_us median  1.5  / p95  3.0  (1 chunk = 1 glDrawElements)
- Modern:  cpu_us median  6.4-7.0 / p95  9-14 (51 visible LBs, 1 MDI call)

Modern is ~4× slower on CPU at radius=5 because the chunked legacy path
already collapsed the scene to one draw call. The architectural wins
(zero glBindTexture/frame; constant-cost dispatch as A.5 raises radius)
will be documented in T10's perf baseline doc; the spec's
"≥10% lower CPU" acceptance criterion is invalid at radius=5 and needs
revision.

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

src/AcDream.App/Rendering/Wb/BindlessSupport.cs

@@ -45,14 +45,14 @@ public sealed class BindlessSupport
             _ext.MakeTextureHandleNonResident(handle);
     }
 
-    /// <summary>
-    /// Set a sampler-typed uniform from a 64-bit bindless handle. Uses
-    /// glProgramUniformHandleARB so it doesn't require the program to be bound.
-    /// </summary>
-    public void SetSamplerHandleUniform(uint program, int location, ulong handle)
-    {
-        _ext.ProgramUniformHandle(program, location, handle);
-    }
+    // Phase N.5b note: a `SetSamplerHandleUniform` wrapper was added in T6
+    // and removed when terrain rendering surfaced GL_INVALID_OPERATION on
+    // NVIDIA Windows for the `uniform sampler2DArray` + glProgramUniformHandleARB
+    // combination. The replacement pattern (uvec2 handle uniform + GLSL
+    // sampler-from-handle constructor — see terrain_modern.frag) lives at the
+    // call site via plain `_gl.ProgramUniform2(program, loc, low, high)`. If
+    // you re-introduce a sampler-handle helper, restrict it to drivers known
+    // to accept the direct sampler-uniform path.
 
     /// <summary>Detect <c>GL_ARB_shader_draw_parameters</c> in addition to bindless.
     /// N.5's vertex shader uses <c>gl_BaseInstanceARB</c> and <c>gl_DrawIDARB</c>