enhance: layered (composable) filter toggles, highlighted rows, side panel next to image

Filters are now independent toggles that compose: the active set is a
bitmask (u_enhance_mask) and the enabled GEGL ops are chained in array
order (enhancer_apply_chain / enhancer_export_chain). Press 1-8 to
toggle any preset on/off at any time; 0 / the Original row clears all;
the live preview re-applies the whole stack each time, so you can
add/remove filters in any order. The window title lists the enabled
presets.

The side panel now sits NEXT TO the image (a horizontal content box:
[panel] + image area) instead of over it; in fullscreen it reparents
to an overlay over the image so fullscreen stays full. Enabled rows
are highlighted (.ggaze-enhance-on: accent background, white bold).

enhancer.c: extract _op_for_builtin, add enhancer_apply_chain and
enhancer_export_chain (single-preset apply/export kept for the tests);
_save_buffer factored out (ju0/ku0 fixes unchanged). New
test /enhancer/apply_chain (compose + empty-mask rejection).

0 files changed, 0 insertions, 0 deletions