phase(N.5) Task 5 fixup: shader doc + extension cleanup

Code quality review caught four issues:
- Unnecessary GL_ARB_bindless_texture extension in mesh_modern.vert
  (vert doesn't use bindless types). Removed; only the frag needs it.
- SSBO binding=1 (BatchBuffer) and UBO binding=1 (SceneLighting) are
  in distinct GL namespaces — added a comment in the vert documenting
  this so Task 10's bind site doesn't get confused.
- Misleading "0=opaque, 1=transparent" comment expanded to spell out
  the full Decision 2 two-pass alpha-test logic and what each discard
  threshold protects against.
- BatchData.flags field is reserved; documented that N.5's dispatcher
  owns all blend state, with a hook for future shader-side additive.

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

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

@@ -7,7 +7,13 @@ in vec3 vWorldPos;
 in flat uvec2 vTextureHandle;
 in flat uint  vTextureLayer;
 
-// 0 = opaque (discard alpha<0.95), 1 = transparent (discard alpha>=0.95)
+// uRenderPass values (Phase N.5 Decision 2 — two-pass alpha-test):
+//   0 = opaque pass     — discard fragments with alpha < 0.95
+//                         (lets the depth write succeed for solid pixels)
+//   1 = translucent pass — covers AlphaBlend / Additive / InvAlpha;
+//                         discard alpha >= 0.95 (already drawn opaque) and
+//                         alpha < 0.05 (skip empty fragments — large
+//                         transparent overdraw cost otherwise)
 uniform int uRenderPass;
 
 // SceneLighting UBO — IDENTICAL layout to mesh_instanced.frag binding=1.