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);