Implement Charts

[ddimaria]

Summary

Adds chart reading to the XLSX reader via two new methods:

let charts: Vec<Chart> = workbook.worksheet_charts("Sheet1")?;
let charts = workbook.worksheet_charts_at(0);

The reader walks the worksheet (or chartsheet) relationships to the drawing part, collects the anchored chart frames (two-cell, one-cell and absolute anchors, including their positions), and parses each chart part into a public Chart data model in the new chart module. The API is modelled after rust_xlsxwriter's chart types so the two crates feel symmetrical (write with rust_xlsxwriter, read back with calamine).

Also exposes the workbook theme palette:

let theme: ThemeColors = workbook.theme_colors();
let series_color = theme.accents()[0]; // Excel's default color for an unstyled first series

ThemeColors has named slots (dark1/light1/dark2/light2, accent1–accent6, hyperlink, followed_hyperlink), parsed once from xl/theme/theme1.xml, cached and reused by chart/style/conditional-formatting parsing, and falling back to the default Office palette (never errors). Consumers need it to color unstyled chart series the way Excel does (the theme accent cycle) without re-opening the package themselves.

Supported

• All classic ECMA-376 chart families: bar, column, line, pie, doughnut, pie-of-pie/bar-of-pie, area, scatter, radar, stock, bubble and surface — including stacked, percent-stacked and 3D variants (with standard vs clustered 3D grouping distinguished), wireframe/contour surfaces, and combo charts (multiple plot groups per chart).
• Excel 2016+ chart-ex parts (xl/charts/chartExN.xml): funnel, treemap, sunburst, histogram, pareto, box & whisker, waterfall and filled map, including the mc:AlternateContent drawing anchors, the chartEx relationship type, and per-series layout options (ChartExLayout: histogram binning, waterfall subtotals/connector lines, box & whisker statistics and element visibility, treemap parent label layout).
• Series: range formulas plus cached values, multi-level (hierarchical) category caches with all levels preserved, names, per-point formatting, markers, smoothing, pie explosion, data labels (including per-point c:dLbl overrides with delete/custom text/position), trendlines (all six types), error bars, invert-if-negative.
• Axes: category/value/date/series types, position, min/max, log base, reversed, units, tick marks, tick-label position, crossing, crossing partner (c:crossAx), cross-between, display units, gridlines, number formats, titles, fonts and label text rotation.
• Chart/group options: titles (rich text and formula-linked), legends, 3D view settings (rotX/rotY/perspective/depth, per Correct MBSC to MBCS in vba.rs #180 in rust_xlsxwriter), gap/overlap, hole size, first slice angle, bubble scale/size-represents, pie-of-pie split settings (including c:custSplit point lists), up/down bars, drop lines, high-low lines, series lines, data tables, dispBlanksAs, and the chart-space flags autoTitleDeleted/plotVisOnly/showDLblsOverMax/date1904.
• Formatting: solid/gradient/pattern fills (with linear gradient angle), line width/dash/color, fonts, with DrawingML scheme-color resolution against the workbook theme (including lum/tint/shade modifiers).
• Anchors: ChartPosition carries the anchor cells/extents plus editAs (move/size behavior) and the absolute anchor x/y position.

Not read (documented in the module docs)

Pivot chart sources, manual plot-area layouts, and surface band formats.

Test plan

• tests/charts.xlsx fixture (pre-generated, committed to tests/ per the feedback on feat(xlsx): implement cell style extraction with rich text and worksheet layout #653 — no generator or dev-dependency added) with 18 classic charts + 5 chart-ex charts exercising every family and the options above.
• 36 chart integration/unit tests, including exhaustive resolve_chart_type mapping tests, theme palette tests against both a default-theme and a real Excel theme, and a real Excel-generated chartsheet (tests/issue438.xlsx).
• Full suite passes with --all-features; clippy and rustdoc clean.

Notes

Stacked on #653 (styles) and #628 (conditional formatting); the first two commits here are those PRs. Only the feat(xlsx): implement chart reading commit is new — happy to rebase once the parents land.