Drop stale T10 follow-up note in toplevel_export.rs

The T10 follow-up block (lines 561-575) said gbm_bo_map was a
documented follow-up. T10 is shipped (commit 0ba3c59, real pixel
read via runtime dlopen of libgbm.so.1); this comment block is now
stale and misleading.

Also fix the misleading "Public stub: where the gbm_bo_map read
belongs" docstring on read_pixels_via_gbm -- the function delegates
to the real implementation now, not a stub.

cargo test 98+/0; clippy clean.

src/toplevel_export.rs

@@ -558,23 +558,10 @@ impl Dispatch<wl_buffer::WlBuffer, ()> for ExportState {
     }
 }
 
-// ---- T10 follow-up note ----
-//
-// The actual `gbm_bo_map` (import dmabuf → gbm_bo → map → read pixels →
-// copy into a PNG-encoded buffer) is a documented follow-up. Wiring it
-// in is one new dep (`gbm` + `drm-fourcc` + a DRM device handle) plus a
-// ~50-line renderer that:
-//   1. opens /dev/dri/renderD128,
-//   2. creates a gbm_device,
-//   3. imports the dmabuf fd (we have it from the `object` event),
-//   4. gbm_bo_map(...) → *mut u8,
-//   5. reads stride*height bytes, swizzles into the PNG writer above.
-//
-// Until then `write_synthetic_frame` produces the right-sized, format-
-// labelled placeholder so callers can verify the wire path.
-
-/// Public stub: where the gbm_bo_map read belongs. Kept as a function
-/// so the test below can assert its shape.
+/// Read the plane-0 dmabuf as an RGBA8 byte buffer and write a PNG
+/// of it to `dest`. This is the public entry point used by callers
+/// that want the raw pixels (the live `capture_toplevel` path goes
+/// through `write_pixels_via_gbm` instead).
 pub fn read_pixels_via_gbm(frame: &DmabufFrame, dest: &Path) -> anyhow::Result<Vec<u8>> {
     read_pixels_via_gbm_full(frame, dest)
 }