feat(A.5 T20): MSAA 4x + alpha-to-coverage on foliage

Per Phase A.5 spec §4.9.2: ClipMap foliage uses binary alpha-cutoff.
At N₂=12 horizon distance the pixel-stepped silhouettes are visible.
A2C with MSAA 4x produces smooth retail-faithful tree edges.

GL context now requests Samples=4. WbDrawDispatcher's opaque pass
toggles GL_SAMPLE_ALPHA_TO_COVERAGE on/off around the multi-draw
indirect call. mesh_modern.frag's opaque pass now discards only
truly-empty (α<0.05) so the GPU derives sample mask from coverage;
transparent pass boundary logic is unchanged.

MSAA audit: no custom FBOs found — all rendering uses default
framebuffer. Sky/particles/ImGui are all MSAA-compatible.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -830,6 +830,7 @@ public sealed class GameWindow : IDisposable
                 ContextFlags.ForwardCompatible,
                 new APIVersion(4, 3)),
             VSync = false,  // off during development so the perf overlay shows true framerate
+            Samples = 4,    // A.5 T20: MSAA 4x for A2C foliage smoothing
         };
 
         _window = Window.Create(options);

src/AcDream.App/Rendering/Shaders/mesh_modern.frag

@@ -80,8 +80,11 @@ void main() {
     vec4 color = texture(tex, vec3(vTexCoord, float(vTextureLayer)));
 
     // Two-pass alpha-test (N.5 Decision 2).
+    // A.5 T20: opaque pass writes alpha as-sampled so GL_SAMPLE_ALPHA_TO_COVERAGE
+    // derives the MSAA sample mask from it — ClipMap foliage edges become smooth.
+    // Discard only fully-transparent (α < 0.05); the GPU handles coverage masking.
     if (uRenderPass == 0) {
-        if (color.a < 0.95) discard;        // opaque pass
+        if (color.a < 0.05) discard;        // opaque pass — kill truly empty only (A2C)
     } else {
         if (color.a >= 0.95) discard;       // transparent pass
         if (color.a < 0.05) discard;        // skip totally-empty

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

@@ -488,6 +488,10 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
         {
             _gl.Disable(EnableCap.Blend);
             _gl.DepthMask(true);
+            // A.5 T20: enable A2C for ClipMap foliage — GPU derives sample mask
+            // from the alpha written by mesh_modern.frag so foliage edges are
+            // smooth under MSAA 4x. A no-op for fully-opaque (α=1) batches.
+            _gl.Enable(EnableCap.SampleAlphaToCoverage);
             _shader.SetInt("uRenderPass", 0);
             _gl.BindBuffer(BufferTargetARB.DrawIndirectBuffer, _indirectBuffer);
             if (diag && _gpuQueriesInitialized) _gl.BeginQuery(QueryTarget.TimeElapsed, _gpuQueryOpaque);
@@ -498,6 +502,7 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
                 (uint)_opaqueDrawCount,
                 (uint)DrawCommandStride);
             if (diag && _gpuQueriesInitialized) _gl.EndQuery(QueryTarget.TimeElapsed);
+            _gl.Disable(EnableCap.SampleAlphaToCoverage);
         }
 
         // ── Phase 8: transparent pass ────────────────────────────────────────