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/TerrainModernRenderer.cs

@@ -50,9 +50,9 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
     private uint _indirectBuffer;
     private int  _indirectCapacity;
 
-    // Cached sampler-uniform locations (matrix uniforms are set by name via Shader.SetMatrix4).
-    private int _uTerrainLoc;
-    private int _uAlphaLoc;
+    // Cached uvec2-handle uniform locations (matrix uniforms are set by name via Shader.SetMatrix4).
+    private int _uTerrainHandleLoc;
+    private int _uAlphaHandleLoc;
 
     // Reusable per-frame buffers.
     private readonly List<int> _visibleSlots = new();
@@ -77,8 +77,8 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
         _alloc = new TerrainSlotAllocator(initialSlotCapacity);
         _slots = new SlotData?[initialSlotCapacity];
 
-        _uTerrainLoc = _gl.GetUniformLocation(_shader.Program, "uTerrain");
-        _uAlphaLoc   = _gl.GetUniformLocation(_shader.Program, "uAlpha");
+        _uTerrainHandleLoc = _gl.GetUniformLocation(_shader.Program, "uTerrainHandle");
+        _uAlphaHandleLoc   = _gl.GetUniformLocation(_shader.Program, "uAlphaHandle");
 
         _globalVao = _gl.GenVertexArray();
         _globalVbo = _gl.GenBuffer();
@@ -234,8 +234,13 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
         _shader.SetMatrix4("uProjection", camera.Projection);
 
         var (terrainHandle, alphaHandle) = _atlas.GetBindlessHandles();
-        _bindless.SetSamplerHandleUniform(_shader.Program, _uTerrainLoc, terrainHandle);
-        _bindless.SetSamplerHandleUniform(_shader.Program, _uAlphaLoc,   alphaHandle);
+        // Pass each 64-bit handle as a uvec2 (low 32 bits, high 32 bits).
+        // GLSL constructs sampler2DArray(uTerrainHandle) at the use site —
+        // see terrain_modern.frag for why this is the safe pattern.
+        _gl.ProgramUniform2(_shader.Program, _uTerrainHandleLoc,
+            (uint)(terrainHandle & 0xFFFFFFFFu), (uint)(terrainHandle >> 32));
+        _gl.ProgramUniform2(_shader.Program, _uAlphaHandleLoc,
+            (uint)(alphaHandle   & 0xFFFFFFFFu), (uint)(alphaHandle   >> 32));
 
         _gl.BindVertexArray(_globalVao);
         _gl.MemoryBarrier(MemoryBarrierMask.CommandBarrierBit);