Workspace capture writes a readable image, and left-edge resize holds its anchor

Two things, and the first is smaller than I told anyone.

WORKSPACE CAPTURE. I said off-screen workspace capture did not exist and
would need building. It already did: udev/capture.rs renders a workspace
that is not on screen, at the target monitor's native size, downscaled to a
requested size, wallpaper included. Verified on the live DRM session rather
than from the source - capturing the active workspace and a non-visible one
gave 320x180 images with mean luminance 0.067 and 0.137, so the second is
genuinely a different render and not a copy of what is presented.

The only thing missing was the container. It wrote PPM, which the shells
that want thumbnails cannot decode, so the file was written successfully,
returned successfully, and silently not drawn - the same failure class as a
capture pass that omits a tier. encode_capture now picks the format from the
destination's extension: .ppm still writes PPM so existing callers keep
working, .jpg/.jpeg write JPEG, anything else writes PNG. Four tests check
the actual magic bytes rather than trusting the call, plus the unfamiliar
extension fallback and a size-mismatch error.

LEFT-EDGE RESIZE. Reported as content resizing "from the right side even
when i resize from left". The window's origin moves the instant the pointer
does, but the client only commits a matching buffer some frames later, so
its still-old content was being placed at the new origin - which slides the
whole window rather than growing it, and leaves the edge that should be
nailed down drifting.

sync_geometry now derives the origin from the size the client has actually
committed when the drag is from a left or top edge, so the opposite edge
stays exactly where the drag started and the dragged edge catches up as
commits arrive. A right or bottom drag is untouched: its origin never moves.

533 tests pass, clippy clean.

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