Remove resolved corner-rounding diagnostics

The top-rounds-but-bottom-doesn't investigation these were tracking is
closed: live-verified via a real screenshot (pixel-level, not
eyeballed) that both corners round correctly on both a decorated
window and an undecorated/CSD one relying on content masking. Removes
three log::debug! blocks (corner-mask state, TOP/BOTTOM border strip
position dumps, and a raw alpha-byte dump of the bottom border buffer)
that were firing on every single render pass regardless of whether
anything changed, adding real per-frame overhead for output no longer
needed. Build + clippy + test (378 passing) all still green.

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