egui037: Integrate DesignCraft main EPUB and PDF updates

docs/target-app-parity.md

@@ -394,12 +394,14 @@ It is evidence of breadth: each item exists. It is not a claim of InDesign-level
 - View ▸ Rotate Spread (`view.rotateSpread`, `layout.rotateSpreadView`): the spread in view turns in quarter turns on screen (stored with the document and in IDML's spread transform; output is unaffected). The canvas maps each spread through its own transform, so tools, snapping, resize handles, guides, overlays and rendering all follow the turned spread.
 - Vertical type (Vertical Type Tool, Type ▸ Story Direction `type.storyDirection`, IDML StoryOrientation): the direction belongs to the story, so every frame of a thread is vertical or horizontal together and a frame threaded onto a story takes its direction (`object.textFrameOptions {vertical}` sets the frames' stories); lines run top to bottom and follow each other right to left; the arrow keys follow them (Down and Up along the line, Left and Right to the next and previous line, keeping the position along it); CJK characters stay upright and other text turns, on screen and in PDF; CJK line breaking with basic kinsoku (also in horizontal text); Korean breaks at spaces, word by word (Hangul in any language, hanja in Korean text), or between syllables when the paragraph asks for it (`type.para {koreanCharBreaks}`, Paragraph Style Options ▸ Justification; not written to IDML). Upright text is shaped top to bottom: punctuation and brackets take their vertical forms (OpenType `vert`; `vkrn` and `vpal` through OpenType features), glyphs advance by their vertical metrics (`vmtx`) and hang from their vertical origins (`VORG`) on the centre of the ideographic em box (`BASE`, else OS/2), which the first line's ascent follows; turned text keeps its horizontal forms. Tate-chu-yoko (Type ▸ Tate-Chu-Yoko, `type.tateChuYoko`, IDML `Tatechuyoko`) sets a run such as "12" side by side within one em of the line. Ruby (Type ▸ Ruby…, `type.ruby`, IDML RubyString; group ruby spread 1-2-1 or centred) and kenten emphasis dots (`type.kenten`, IDML KentenKind) are set over their text — to its right in vertical frames — without changing line breaks. Warichu and ruby options (per-character ruby, positions, overhang) aren't there yet.
 - Math expressions (Type ▸ Insert Math Expression…, `math.insert`, `math.svg`): a LaTeX subset (fractions, scripts, roots, big operators with limits, Greek, relations and operators with math spacing, `\left…\right`, `\text`) typeset by our own engine into SVG, placed inline at the cursor or as a frame; the LaTeX is kept as alt text and can be edited.
-- Object Layer Options for placed PDFs (`object.pdfLayers`, `object.layerOptions`, Object ▸ Object Layer Options…): the file's optional-content layers listed and shown or hidden per frame on screen; in exported PDFs such graphics go out as 300 ppi images of their visible layers.
+- Object Layer Options for placed PDFs (`object.pdfLayers`, `object.layerOptions`, Object ▸ Object Layer Options…): the file's optional-content layers listed and shown or hidden per frame on screen. Plain and PDF/X-4 exports retain vector artwork and optional content with the source defaults or per-frame overrides, including independent states for repeated placements. Missing or unreadable default configurations and unsupported base states fall back to all-ON defaults with a file-named warning, preserving explicit frame overrides. Readable groups omitted from `/OCGs` are added with a distinct file-named warning, preserving their default ON/OFF states; concurrent recovery warnings are combined. Malformed ON data is recoverable only when a known ON base and empty OFF list prove it cannot hide artwork; missing display names are generated without changing known visibility when no override needs name matching. Contradictory states, unresolved references and ambiguous name overrides remain errors. PDF/A-2b command exports retain the existing 300 ppi raster path for explicit overrides; PDF/A-2 permits optional content, but this change does not extend archival validation. IDML GraphicLayer visibility import remains unsupported.
+- Placed-PDF export preparation is cached per asset and hidden-name set; repeated placements retain recovery warnings (identical warnings are deduplicated). When all groups are visible and named, the original source bytes are embedded without an incremental layer update, including sources whose broken cross-reference tables the reader repairs. Plain, PDF/X-4, booklet and book-merge output register those unmarked groups as visible. Hidden groups or generated names still require a usable cross-reference table; damage errors identify the placed file, and export retains the first placement error rather than replacing it with a later failure.
 - Effects: Inner Glow (edge or centre), Bevel and Emboss (inner bevel lit from the light angle), Satin (`object.innerGlow`, `object.bevel`, `object.satin`); PDF export now keeps soft effects: objects with shadows, glows, feathers, bevels or satin go out as 300 ppi transparent images of their appearance (threaded text frames excepted).
 - Directional Feather (`object.directionalFeather {widths: [top, left, bottom, right]}`): each side fades over its own width.
 - View ▸ Flattener Preview (`view.flattenerPreview`): objects involving transparency highlighted in red; knockout groups export as images (PDF output has no knockout groups).
 - Live captions (`object.caption {live: true}`, Object ▸ Captions ▸ Generate Live Caption): the caption text is re-filled from the source object's metadata after every edit (same undo step).
-- Style Export Tagging (`style.exportTag`, Paragraph Style Options ▸ Export Tagging): paragraph and character styles choose their HTML element and class in EPUB/HTML.
+- Style Export Tagging (`style.exportTag`, Paragraph Style Options ▸ Export Tagging): paragraph and character styles choose their HTML element and class in EPUB/HTML. Its Split Document (EPUB only) flag marks the styles that start a new EPUB content file when the export splits on export tags.
+- EPUB Export Options › Split Document (`document.epubSplit`, File › Export EPUB… dialog; `file.exportEpub {split}` for one export): off (one content file, the default), before each paragraph of a single paragraph style, or before each paragraph whose style has Split Document on in its Export Tagging. Navigation and note links point across the files. The setting is stored with the document, follows a renamed style, and round-trips through IDML (`EPubExportPreference`); Load Styles carries the per-style flag.
 - Tagged Text: export (`file.exportText {format: "tagged"}`) and place (detected by its header; ASCII, UTF-8 or UTF-16) with paragraph and character styles, font, typeface and size, and `<0x…>` characters.
 - Tagged PDF: one structure element per paragraph (H1–H6 and BlockQuote from Export Tagging, else P) in reading order, frame fills, strokes, rules and tables as artifacts, figures with alt text.
 - EPUB: cover image from the first page (`file.exportEpub {cover}`), and Fixed Layout export (`file.exportFixedEpub`, File ▸ Export EPUB (Fixed Layout)…): pre-paginated pages as 144 ppi images with their text kept for search and read-aloud.