feat(odf): draw a custom shape as the shape, not as its box

[andiwand]

🤖 Generated with Claude Code

Stage 4 of #771, on main now that #798 has landed. Closes #159.

html::translate_custom_shape carried a // TODO draw shape in svg and emitted
a positioned <div> with fill and stroke — the bounding box, never the shape.
Every one of the corpus's 350 custom shapes came out a rectangle, arrows,
callouts, rings and stars alike.

read_path now resolves a draw:custom-shape's draw:enhanced-geometry
through the languages #798 added: svg:viewBox gives the box the formulas
measure against, draw:modifiers fills $0, $1, …, the draw:equation
children resolve on demand and are memoised, with a reference that cycles
resolving to nothing rather than recursing, and draw:enhanced-path becomes
the svg d.

Notes

• draw:mirror-horizontal and draw:mirror-vertical are folded into the
  coordinates as they are written, rather than into a transform: the pen keeps
  tracking the unmirrored geometry, so the angles and radii an arc needs are
  computed once, and only the emitted numbers and the sweep flags are
  reflected.
• The path is drawn with fill-rule="evenodd", which is what leaves a ring its
  hole; the corpus has one.
• hasstroke and hasfill evaluate as true because the geometry reader has no
  style in hand — no corpus formula reads them.

Not done, and recorded in PLAN.md

draw:text-areas, so a shape's text is still laid out in the whole box;
draw:handle, which only an editor needs; and F/S, which want one subpath
painted differently from the rest.

Verification

Checked against libreoffice's own png export of sample.odg and
SpotfaceCoord40270-L.odg.

The reference-output pin is advanced: 36 files, every changed line an insertion
— 740 <svg><path> pairs where a custom shape used to be a plain box, and
nothing removed.