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.vert

@@ -1,5 +1,4 @@
 #version 430 core
-#extension GL_ARB_bindless_texture : require
 #extension GL_ARB_shader_draw_parameters : require
 
 layout(location = 0) in vec3 aPosition;
@@ -17,13 +16,23 @@ struct InstanceData {
 struct BatchData {
     uvec2 textureHandle;   // bindless handle for sampler2DArray
     uint  textureLayer;    // layer index (always 0 for per-instance composites)
-    uint  flags;           // reserved
+    uint  flags;           // reserved — N.5 dispatcher owns all blend state
+                           // (glBlendFunc per pass). If a future phase wants
+                           // shader-side per-batch additive flag (Decision 2
+                           // fallback), encode it here as bit 0.
 };
 
 layout(std430, binding = 0) readonly buffer InstanceBuffer {
     InstanceData Instances[];
 };
 
+// binding=1 here is the SSBO namespace — distinct from the UBO namespace.
+// SceneLighting UBO also uses binding=1 in the fragment shader; GL keeps
+// GL_SHADER_STORAGE_BUFFER and GL_UNIFORM_BUFFER binding tables separate.
+// Task 10 dispatcher binds:
+//   glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, instanceSsbo)
+//   glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 1, batchSsbo)
+// Existing SceneLightingUboBinding handles the UBO side.
 layout(std430, binding = 1) readonly buffer BatchBuffer {
     BatchData Batches[];
 };