[mkroemer]

Builds on the pictures PR (#11). Until it is merged this PR shows its commit too; the charts part is the commits after it.

This draws a spreadsheet's charts instead of the placeholder box, from the live cells, in the look the file gives them. It is large (about 2,250 lines, 315 of them tests), but nearly all of it is two new files: src/core/xlsx/chart.ts reads a chart part into a model in the worker, and src/core/xlsx/chart-svg.ts draws that model as a self-contained SVG with no DOM, shown over the cells like a picture. The commits split it into the reader and drawing, review fixes, the looks, and the last four kinds, if you'd rather review it in steps; I can also split it into separate PRs along those lines.

• Kinds: bar and column (clustered, stacked, 100%, upright or horizontal), line (markers, smoothing, gaps for empty cells), area, pie and doughnut (exploded slices, first-slice angle), scatter, bubble (by area or width, bubbleScale, negative bubbles), radar (lines, markers, filled), stock (high-low lines, up-down bars), surface seen from above as Excel's contour chart or as a wireframe, and combinations on primary and secondary axes; 3D charts as their flat form. Only the chartex kinds (waterfall, treemap, ...) keep the placeholder.
• Values: series references are read from the workbook's cells, loading a sheet not shown yet, so charts follow edits. A defined name, another workbook or an unreadable reference falls back to the chart's cached values. Hidden rows and columns are left out when plotVisOnly is set.
• Layout: Excel-like automatic axis bounds (zero kept, steps of 1/2/5, a 100% axis ending at 100%); fixed min, max and major unit; reversed and log axes; gridlines; tick and data labels in their number formats, tilted where they don't fit flat; the title (Excel's automatic one included, and a title taken from a cell); the legend (positions, overlay, deleted entries); manual plot layout.
• Look: series, point, chart area and plot area fills as colors, gradients (linear, radial) or pattern fills (the DrawingML presets); line colors, widths and dashes; markers; the text size, weight and color of the title, axes, legend and data labels; gridline and axis lines or none; the built-in chart style (c:style, c14:style: its palettes, tinted plot areas and dark backgrounds, not bevels and shadows). Where the file gives nothing, the grays are shades of the theme's text color, as Excel derives them.
• Pie and doughnut size a negative value by its magnitude, as Excel does, and keep the sign in its label. In a scatter chart a point missing its X value is not drawn. Everything plotted is clipped to the plot area, so a fixed axis range can't run over the title and legend. Each chart is its own <img>, so gradient, pattern and clip ids only need to be unique within one picture.

test/xlsx-chart.test.ts covers series read from the cells and the cache fallback, hidden rows, a chart read again after an edit, Excel's axis bounds and the 100% axis, column, stacked bar, line and pie drawings, cell titles, negative pie values, scatter gaps, clipping, each of radar, bubble, stock and surface, gradient and pattern fills, text and line colors, the plot area fill, the theme's grays and the built-in styles. bun run typecheck, bun test and the e2e suite pass in CI.

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