PDF: draw horizontal and vertical lines inside groups

[cyril265]

Problem: a horizontal or vertical line inside a group is missing from the exported PDF, although the screen shows it. Word's documents use this for dividers inside grouped boxes (for example a line between two text blocks in an info box), so the PDF loses them.

Cause: such a line keeps a zero height or width in the group (a:ext cy="0"). shape_path in crates/pdf/src/lib.rs returned None for every shape with w <= 0 || h <= 0, so the PDF export skipped it. The raster renderer has no such check, so the screen drew it. A floating line on its own was not affected, because float_rect gives it a 1 pt frame; connector presets (straightConnector1 and the others) were not affected either, because they are drawn as paths. The affected shape is prst="line", which Word also writes for its straight connectors (with wps:cNvCnPr).

Fix: lines (and connectors, for the frame-based fallback) may now be flat on one axis. Closed shapes still need an area, and a line without length is still skipped.

Found while converting generated letters to PDF with wordcraft-cli convert.

Reproduction: lines.docx (hand-written from the spec, no Word output), then wordcraft-cli convert lines.docx out.pdf: the red divider inside the grey group is missing on main.

Both images: wordcraft-cli convert lines.docx out.pdf, then pdftoppm -r 100. lines.docx is a hand-written fixture: a group (wpg:wgp) holding a rounded rectangle and a prst="line" shape with a:ext cy="0".

Note: the 1 pt minimum in float_rect still draws a standalone horizontal line as a 1 pt diagonal; left alone here.

docs/file-format-parity.md and the ROADMAP.md progress log are updated.

Tests

New and changed tests in crates/pdf/src/tests.rs:

• flat_lines_in_a_group_are_drawn: a group with a horizontal, a vertical and a zero-length line exports exactly two more paths than the empty group.
• shapes_have_paths: horizontal and vertical lines and connectors get a path; a flat rectangle, a line without length, and NaN, infinite or negative sizes don't.

Before the fix:

thread 'tests::flat_lines_in_a_group_are_drawn' panicked at crates/pdf/src/tests.rs:607:5:
assertion `left == right` failed: the horizontal and the vertical line, not the one without length
  left: 0
 right: 2

After: cargo test -p wordcraft-pdf: 26 passed.

cargo fmt --check and cargo clippy -p wordcraft-pdf --all-targets -- -D warnings are clean. I didn't run the wasm build.

[echelon]

Thanks for the contribution, @cyril265! This has landed in main. If you haven't already, come join us on Discord: https://discord.gg/artcraft