Print color: export overprint as /op, /OP and /OPM

Overprint Fill, Overprint Stroke, Overprint Gap, overprinting text (ruby,
kenten) and 100% [Black] under Appearance of Black are pushed through
Surface::push_overprint with OPM 1, around the paint they apply to.
PDF/A exports are RGB and leave overprint out with a warning.

docs/target-app-parity.md

@@ -307,7 +307,7 @@ It is evidence of breadth: each item exists. It is not a claim of InDesign-level
 - Control panel scale X/Y, rotation and shear fields (`transform.info`, absolute `transform.set {scaleX, scaleY, rotation, shear}` about the reference point) and Preferences › Transformations are Totals for nested objects and placed graphics.
 - Variable fonts: each named instance is a style (Font menu, shaping with feature variations, metrics, outlines) and embeds at its axis settings in PDF; free axis sliders still to come.
 - Object ▸ Select (first/next above, next/last below, container, content, previous/next in group), full Object ▸ Convert Shape (`object.convertShape`), Table ▸ Split Cell Horizontally/Vertically.
-- Attributes panel (`object.attributes`: Overprint Fill/Stroke/Gap, Nonprinting), with Overprint Gap in Overprint Preview and IDML.
+- Attributes panel (`object.attributes`: Overprint Fill/Stroke/Gap, Nonprinting), with Overprint Gap in Overprint Preview and IDML; PDF export writes them as `/op` and `/OP` with `/OPM 1`.
 - Page Numbering View (Preferences: section or absolute): page labels everywhere follow it; Go to Page takes section names, numbers or `+n`.
 - Preferences › Advanced Type (document `advancedType`: superscript/subscript size and position; IDML TextPreference); subscripts now drop 33.3% like InDesign.
 - Preferences › Composition › Highlight: keep violations, H&J violations (three shades), custom tracking/kerning, substituted fonts.
@@ -323,7 +323,7 @@ It is evidence of breadth: each item exists. It is not a claim of InDesign-level
 - Place Excel workbooks (.xlsx): the first worksheet's used range becomes a table (shared/inline strings, numbers, booleans).
 - Object Library (`library.new/open/add/place/remove/list/json/close`, Library panel): `.dclib` files of snippets that keep stories, styles, swatches and images.
 - Ink Manager (`ink.list`, `ink.options`; Swatches panel menu): All Spots to Process, per-ink conversion and ink aliases in PDF output; IDML ConvertToProcess/AliasInkName.
-- Appearance of Black complete: Printing/Exporting rich black for RGB output (PNG), Overprint [Black] Swatch at 100% (document, IDML OverprintBlack).
+- Appearance of Black complete: Printing/Exporting rich black for RGB output (PNG), Overprint [Black] Swatch at 100% (document, IDML OverprintBlack; 100% [Black] fills, strokes and text overprint in PDF export).
 - Hyperlinks and Bookmarks panels (Window ▸ Interactive): new from the selection, edit (`hyperlink.edit`), go to source (`hyperlink.goToSource`), rename bookmarks (`bookmark.rename`), go to page, delete.
 - Nested line styles (`nestedLineStyles` paragraph attribute, Paragraph Style Options › Drop Caps and Nested Styles, IDML AllNestedLineStyles): the first lines restyle until the line breaks settle.
 - Drop caps (`type.para {dropCapLines, dropCapChars, dropCapStyle, dropCapAlignLeft, dropCapScaleDescenders}`, Paragraph panel, Paragraph Style Options › Drop Caps and Nested Styles; IDML DropCapLines/DropCapCharacters/DropcapDetail, the drop cap style as the leading "through 1 Dropcap" nested style): the first characters (code points, as InDesign counts them, never splitting a letter from its combining marks; up to 150) are enlarged from the first line's cap height down N − 1 leadings of the text after them (up to 25 lines; grid steps on a baseline grid), the lines beside them are indented by their advance (on the right in right-to-left paragraphs), spaces right after them are set at their size without widening the indent, baselines don't move; a one-line drop cap keeps a smaller size of its own and sets its tabs at the paragraph's tab stops; Align Left Edge (on by default) moves the drop cap left by its first letter's side bearing so its ink starts at the indent; Scale for Descenders is kept but doesn't change the layout yet; a drop cap character style, and nested styles through the drop cap (Dropcap delimiter). Drawn on screen and in PDF (text in order), with caret, hit testing and selection over the enlarged letters. Vertical text sets no drop caps.