feat(xlsx): render embedded charts on chart-only sheets as line plots

[developer0hye]

Summary

• An embedded chart on a sheet with no cells was dropped entirely: the sheet was skipped like XLSX: render worksheet drawings and images #238's image-only sheets, and that fix did not extend to charts. The audited workbook rendered only the source cells
• Emit the sheet's charts on a page from both the streaming and non-streaming paths (extending the empty-sheet handling)
• Charts that omit <c:cat> now get implicit 1..N category labels like Excel
• Line/area charts render as an axis-scaled polyline plot (gridlines, value/category axes, per-series lines with markers, legend) built on PPTX: preserve native chart layout and value formatting #217's axis foundation — instead of the trend-indicator table

Test plan

• New tests: the embedded chart is extracted with its series (integration); the line plot emits polyline paths, gridlines, legend, category labels (codegen). Updated the old line trend-indicator test for the new plot output
• Full workspace suite passes; fmt --check clean; clippy -D warnings clean

Visual verification (public WithChart fixture vs Excel GT, chart page)

Excel GT	Before	After

The two-series line chart (rising blue, dipping orange) renders with gridlines, a 1–6 category axis, and a legend, matching GT. Before, the chart was omitted entirely.

After-image re-audit

Minor accepted differences: value axis rounds to 0–20 (GT uses Excel's 0–18), and series 2 uses the Office accent2 orange (GT's chart XML applies a red). No new issues.

Related: #239

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