Foundation for a per-fill Fill editor (stack multiple fills, add/remove by
row). Additive, backward-compatible (fill is already Vec<PenFill>; old .op
files load as a single fill):
- fills.rs: fill_type_of, fill_count, fill_type_at, add_fill,
remove_fill(i), set_fill_type_at(i), set_fill_hex_at(i),
set_fill_opacity_at(i).
- property_edit_mutators.rs: add_selected_fill, remove_selected_fill(i),
set_selected_fill_type_at(i), set_selected_fill_hex_at(i),
set_selected_fill_opacity_at(i) (via a with_selected_node helper).
UI/host wiring lands separately.
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