Root cause: auto selected the key with MORE kept pixels, which always
picked the green key (it keeps 100% of dark non-green backgrounds).
The black key correctly isolates the slab on dark surfaces.
Fix:
- auto now prefers the key with FEWER kept pixels (removed more bg)
when both are non-empty
- green key has a greenness pre-filter: non-greenish pixels pass
through (kept) so the black key can handle them, instead of the
green key keeping 100% of a dark surface
- added 3 regression tests: dark-bg auto selection, green-sheet auto
selection, green-key passthrough on non-green surfaces
106/106 unit tests green.
Ty