Simplify letterbox fill modes back to Dithered (default), Solid, and
None. Remove the Extend Edges mode and all per-pixel edge replication
code. Restore Bayer ordered dithering for the Dithered fill mode.
Re-introduce edge average caching so cover edge computations persist
across sleep cycles, stored as a small binary file alongside the cover
BMP.
Co-authored-by: Cursor <cursoragent@cursor.com>
ea11d2f7d3
