Part of #372: PDF/.ai import keeps strokes live and hidden and nested layers

[bflatastic]

Opening a PDF-compatible .ai (or any PDF) now keeps each stroked object one path with a live stroke, and brings hidden and nested layers in as hidden layers and sublayers with their art.

• Strokes: Illustrator writes an object as a fill of its outline left open, then a stroke of it closed; the importer only merged identical paths, so every filled-and-stroked object came in as two (a fill and a separate stroke-only path). A fill and a stroke of the same outline (equal once every subpath is closed) now become one path with the fill and a live stroke (width, cap, join, miter limit, dash, paint), taking the stroke's outline. Strokes under a stretched transform stay strokes at their average width (hayro hands strokes over as strokes; nothing outlines them).
• Hidden layers: groups off in the default configuration already came in hidden with their art (hayro skips them, so the importer re-reads the file with every group on). Now a group whose /Usage /View /ViewState is /OFF is hidden too (an /AS view event lets the view state override /D). The interpreter-skip check (skips_any) is kept apart from layer visibility.
• Sublayers: the configuration's /Order nesting (inline or its own object, as Illustrator writes it; labels skipped) makes sublayers in first-paint order, taking their top-level layer's colour. Depth (32), entries and groups are capped; a group listed twice stays where first listed, so the nesting has no cycles.
• Scale: splitting a page's art by group is indexed instead of a linear search per object, so thousands of layers import quickly (test with 3,000, half hidden).
• ROADMAP: shipped line, plus pen tilt/bearing/barrel-rotation brushes (6D art pens) on the gap list and parity row; docs/mcp.md import notes.

Not here: hidden sublayers or objects inside a visible top-level layer aren't in an .ai's PDF part at all (only top-level layers become PDF layers), and brushes/width profiles arrive expanded; both live only in the native private data (out of scope in the ROADMAP).

Checks

• Rebased on 24bea909 (origin/main).
• cargo fmt --all --check, cargo clippy -p vectorcraft-pdf --all-targets -D warnings, cargo test -p vectorcraft-pdf (197 + 1 passed; 5 of the 8 new tests fail without the fix, the other 3 guard behaviour that already held), cargo test -p vectorcraft-engine --test import_fuzz (37), cargo test -p vectorcraft-engine -- pdf (81), cargo xtask layers, cargo xtask brands, cargo xtask wasm, cargo check -p vectorcraft-pdf for x86_64-unknown-linux-gnu and aarch64-apple-darwin.
• Live: vectorcraft-cli run --in <real Illustrator 29 .ai> --cmd document.inspect: the filled-and-stroked object is now one path (fill #343334, stroke #ffffff 1.5) instead of two; the same file with its layer switched off imports as a hidden layer with all its art. Survey of 19 real .ai files: all open, remaining stroke-only paths are genuine open lines.
• MCP stdio: open_file of a nested-layer .ai then inspect_document: layer "Artwork" holds the hidden sublayer "Hidden sublayer" with its art and a fill+stroke path.

🤖 Generated with Claude Code

[bflatastic]

Full cargo xtask ci passed on the integrated stack; merging.