feat(odf): draw the chart an embedded object holds, not its replacement image

[andiwand]

🤖 Generated with Claude Code

Stage 5 of #771, on main now that #799 has landed. Closes #179.

Carries one unrelated commit: Scanner is renamed ValueCursor, since it is
a position in a string that advances rather than anything that tokenises.

A libreoffice chart is
<draw:frame><draw:object xlink:href="./Object 1"/><draw:image xlink:href="./ObjectReplacements/Object 1"/></draw:frame>. draw:object had
no parser, so what drew was the sibling replacement: a flattened metafile with
no selectable text, no structure, and nothing at all where the producer wrote
none. The chart's own markup — series, axes, and a table:table of the plotted
values — sat unread in the package. 1122 draw:object over 21 files in the
test corpus.

odf_chart.cpp renders <chart:chart> to svg: bar, line, area, scatter, pie
and ring, with the title, legend, axes and the series colours the chart's own
styles name. The object reaches the renderer as an image carrying that svg, so
the existing image path writes it out with no renderer change at all.

Two decisions worth recording

• The values come from the part's own local-table rather than the cells it
  names in the host document: that is the snapshot the chart carries, and the
  only source an embedded chart is guaranteed — it is also what lets a chart in
  a text document work, where there is no sheet to read.
• The layout is chart:plot-area's box and chartooo:coordinate-region, so
  the plot sits where the producer put it rather than somewhere we invent.

Notes

• The draw:image beside an object is skipped, or the two would stack; an
  object holding no chart we can read — a formula, an ole blob — falls back to
  it, which is what keeps the 1027 math objects in
  Vektoranalysis Zusammenfassung.odt drawing.
• A bar or an area is measured from zero, or it lies about its size.
• Category labels are thinned to what fits: every nth, n chosen from the widest
  label's width against the plot's.

Open, and recorded in PLAN.md

Stacked and percentage plots, secondary axes, trend lines, data labels, and the
number format an axis names — a date axis shows its serial number today.

Verification

odf_chart_test.cpp, from string literals. style+svm-chart-1.ods and
Blood Pressure.ods were rendered and eyeballed; the latter drew nothing at all
before, having no replacement image.

The reference-output pin is advanced: 19 files, every change a line-for-line
replacement of the metafile replacement image's data uri by the chart's own
svg.