feat(pdf): draw panels as filled Bézier paths instead of rasterizing

[Harsh-2002]

Closes the last item left open from the PDF work. Code block and table backgrounds were flat-colour PNGs — one rasterized per panel at 144 DPI, written to a temp file, decoded, and embedded as an image XObject.

	before	after
README export	200,810 B	192,456 B
image XObjects in it	6	0
code/table heavy fixture	144,520 B	126,725 B
its image XObjects	11	0
temp files written	one per panel	none

Why this needed vendoring genpdfi

Area exposed draw_line — a stroked polyline — and nothing that fills a path. Area.layer is private with no accessor, and Context carries only the font cache, so an Element had no route to printpdf's drawing API, which does support filled Bézier paths.

Added Area::fill_polygon and the Layer::add_filled_shape it calls, both modelled directly on the existing draw_line/add_line_shape pair and going through the same private position()/transform_position() mapping. genpdfi's repository is archived, so there is no upstream to send this to; it is vendored beside printpdf under [patch.crates-io], Apache-2.0 OR MIT, 248 KB.

The output

Real quarter-arc Béziers, k = r × 0.5522847:

0.9529412 0.95686275 0.972549 rg
53.858273 742.1512 m
541.41736 742.1512 l
547.6795 742.1512 552.756 737.07477 552.756 730.8126 c
552.756 723.7378 l
552.756 717.47565 547.6795 712.3992 541.41736 712.3992 c

printpdf reads a curve as four consecutive points, taking the pair marked true as its start and first control — that convention is the fiddly part and is commented at the call site.

What this fixes beyond size

• Exact at any zoom. The raster was quantised at 144 DPI and printpdf sets interpolate: true, so viewers smooth-scaled it — soft and jagged at once.
• No opaque corners. genpdfi rejects images carrying alpha, so a rounded panel could not be transparent outside its arc and had to paint its corners with the page colour. That was invisible on white paper and would have shown as white notches under any PDF theme. The constant introduced for it in fix(pdf): antialias panel corners and give each panel its own temp file #14 is gone.
• No temp files. The per-panel temp file added in fix(pdf): antialias panel corners and give each panel its own temp file #14 to remove a threading hazard is no longer written at all.
• Fewer XObjects to keep ordered for the reproducibility work in fix(pdf): make export byte-reproducible #12.

Unaffected

Real markdown images still embed (verified: ![](pic.png) → 1 image XObject) and mermaid still degrades to a labelled source block without uploading. Output stays byte-identical across runs.

One test updated rather than added: test_export_pdf_renders_code_and_tables asserted a code block produces an image, which is now exactly the wrong contract. It is now test_export_pdf_panels_are_not_rasterized and asserts the opposite. 324 tests pass.