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/GameWindow.cs

@@ -6364,7 +6364,11 @@ public sealed class GameWindow : IDisposable
             else
                 _terrain?.Draw(camera, frustum, neverCullLandblockId: playerLb);
             _terrainCpuStopwatch.Stop();
-            _terrainCpuSamples[_terrainCpuSampleCursor] = (long)(_terrainCpuStopwatch.Elapsed.TotalMicroseconds);
+            // Multiply by 100 then divide by 100 in the diag print to keep
+            // 0.01 µs precision in the long-typed sample buffer. Terrain Draw
+            // is sub-microsecond on simple scenes; truncating to integer µs
+            // would round nearly every sample to 0.
+            _terrainCpuSamples[_terrainCpuSampleCursor] = (long)(_terrainCpuStopwatch.Elapsed.TotalMicroseconds * 100.0);
             _terrainCpuSampleCursor = (_terrainCpuSampleCursor + 1) % _terrainCpuSamples.Length;
             MaybeFlushTerrainDiag();
 
@@ -8778,10 +8782,13 @@ public sealed class GameWindow : IDisposable
         long now = Environment.TickCount64;
         if (now - _terrainLastDiagTick <= 5000) return;
 
-        long cpuMedUs = TerrainDiagMedianMicros(_terrainCpuSamples);
-        long cpuP95Us = TerrainDiagPercentile95Micros(_terrainCpuSamples);
+        // Samples are stored as microseconds × 100 (so 1.23 µs becomes 123 long).
+        long cpuMedHundredthsUs = TerrainDiagMedianMicros(_terrainCpuSamples);
+        long cpuP95HundredthsUs = TerrainDiagPercentile95Micros(_terrainCpuSamples);
+        double cpuMedUs = cpuMedHundredthsUs / 100.0;
+        double cpuP95Us = cpuP95HundredthsUs / 100.0;
         Console.WriteLine(
-            $"[TERRAIN-DIAG{(_useLegacyTerrain ? "/legacy" : "/modern")}] cpu_ms={cpuMedUs / 1000.0:F2}/{cpuP95Us / 1000.0:F2}  " +
+            $"[TERRAIN-DIAG{(_useLegacyTerrain ? "/legacy" : "/modern")}] cpu_us={cpuMedUs:F2}m/{cpuP95Us:F2}p95  " +
             $"draws={_terrain?.VisibleSlots ?? 0}/frame  " +
             $"visible={_terrain?.VisibleSlots ?? 0}  " +
             $"loaded={_terrain?.LoadedSlots ?? 0}  " +