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/Shaders/terrain_modern.frag

@@ -3,8 +3,16 @@
 
 // Phase N.5b: terrain fragment shader on the modern bindless dispatcher.
 // Math identical to terrain.frag (Phase 3c per-cell maskBlend3 +
-// Phase G fog + lightning flash). uTerrain and uAlpha are bound via
-// glProgramUniformHandleARB on the C# side; GLSL sampling is unchanged.
+// Phase G fog + lightning flash).
+//
+// Bindless texture handles are passed as uvec2 (low/high 32 bits) and
+// reconstructed into sampler2DArray at use sites via the GLSL
+// sampler-from-handle constructor. The alternative pattern —
+// `uniform sampler2DArray` set via glProgramUniformHandleARB — produces
+// GL_INVALID_OPERATION on at least one driver in practice (NVIDIA on
+// Windows). The uvec2 + constructor pattern is what N.5's mesh_modern
+// shader uses and is the documented "always works" form per the
+// ARB_bindless_texture spec.
 
 in  vec2  vBaseUV;
 in  vec3  vWorldNormal;
@@ -19,8 +27,10 @@ flat in float vBaseTexIdx;
 
 out vec4 fragColor;
 
-uniform sampler2DArray uTerrain;
-uniform sampler2DArray uAlpha;
+uniform uvec2 uTerrainHandle;
+uniform uvec2 uAlphaHandle;
+#define uTerrain sampler2DArray(uTerrainHandle)
+#define uAlpha   sampler2DArray(uAlphaHandle)
 
 struct Light {
     vec4 posAndKind;