fix(#180): a pushed fill gives a solid face

[octalide]

Closes #180

Plan

A paint's fill_to follows its fill. Every write of a fill through the theme's field layer also writes fill_to with the same value, so a fill alone is a flat face, and fill_to written with it (in any order) or after it makes the gradient.

• theme.follower(r) (fill -> fill_to of the same paint) and theme.follows(r) name the relation once, at the Ref level. put stays the raw single-field write.
• theme.set, set_path and TOML apply write the follower.
• sheet.write (push, push_class) writes and saves the follower, so a pop restores both.
• Operations that write several fields at once write following fields last, so the result does not depend on listing order: push_class takes a second pass, read_into a third (PASS_FOLLOWS).
• save writes fill_to after a written fill whenever the two differ, so a shaded face with a changed fill saves and loads back the same.
• Documented in the theme and style module docs, Paint, the fill_to FIELDS row and the README (field table regenerated to match).

Decisions the issue left open

• Representation: write-time following, not a stored "explicitly set" flag. Paint keeps its layout and the box painter is unchanged.
• The theme editor writes through theme.set, so dragging a fill slider now flattens that paint's fill_to as well, the same rule as every other path.
• The gallery's "Pushed green and round" button is solid through the fix itself (it only showed a gradient under a shaded look like dark). The gallery golden is the last frame of section 0, so it does not change and was not regenerated.

Ran

mach test . --filter style|theme|editor|widget, mach test demo/gallery, mach build . -p release, mach fmt --check on the root and demo/gallery.