perf(render3d): mesh-shader grass dispatches each tile with its own count; not yet counted as implemented

A chunk's dispatch was sized for its largest tile, so the far tiles beside a
near one ran thousands of empty invocations: 9.8 ms of grass at 4K on an RTX
3070 Ti. One dispatch per tile, sized to that tile, brings it to 5.0 ms - still
three times the chunked path's 1.7 (36 fps against 41), so GF_MESH_GRASS is not
implemented yet and the Advanced row says a coming update.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

changes/mesh-grass.md

@@ -1,9 +1,12 @@
 bump: minor
 type: feat
-**Mesh-shader grass** — on a card with `VK_EXT_mesh_shader`, `r3d_mesh_grass(on)` draws each chunk of grass
-tiles as one mesh-shader dispatch, and a blade that is culled emits no vertices at all.
+**Mesh-shader grass** — on a card with `VK_EXT_mesh_shader`, `r3d_mesh_grass(on)` draws each grass tile as one
+mesh-shader dispatch sized to that tile's blades, and a blade that is culled emits no vertices at all.
 
 - **`ludic-dev shaders`** — a variant whose first file is `*.mesh` compiles that stage as a Vulkan 1.3 mesh
   shader; its SPIR-V keeps the `.vert` name, so the manifest is unchanged.
 - **The seam** — `gpu_has_mesh()` and `gpu_draw_mesh_tasks(x, y, z)`; a mesh program's pipeline has no
   vertex input and its bindings the mesh stage. The device enables `meshShader` and `maintenance4`.
+
+Not yet faster: at 4K on an RTX 3070 Ti the grass pass takes 5.0 ms against the chunked path's 1.7, so
+`gpu_feature_implemented(GF_MESH_GRASS)` stays false and a game should not offer it as a finished setting.