Confirm the monitor-seam shadow fix on screen, and retract a wrong lead

The seam bleed the owner reported as "windows show a bit in the other
monitor" was fixed and unit-tested but never seen. With monitor split now
working in a nested instance it can be, and it is.

Same window, same settings, same instance, same scanline. Control, right
edge at x=320 with no seam nearby: a real shadow, (9,9,13) two pixels out,
fading through (11,11,17) and (12,12,19) to the bare desktop (13,13,20) by
23px, a full 24px falloff. Seam, right edge exactly on the boundary at
x=640: (13,13,20) at every sample from two pixels out onward. Nothing
crosses.

Retracting the "SSD versus CSD" lead recorded in the previous commit. The
first attempt at this test looked like a pass until the negative control
failed - the same window off the seam had no shadow either. Narrowing it to
"Alacritty gets a shadow, Nemo does not" pointed at decoration, and that was
written down as a lead. It was neither: one temporary diagnostic in the build
path reported max=true. Nemo restores its own maximized state on startup and
the shadow gate correctly excludes a maximized window, which has no
neighbour to separate from. No bug, and decoration had nothing to do with
it. One log line beat two rounds of reasoning from symptoms; the lead is
retracted in the same file that stated it.

515 tests pass, clippy clean.

0 files changed, 0 insertions, 0 deletions