Fix PPTX chart stalls and render charts as SVG

[xiejinwen113]

Motivation

PPTX charts backed by embedded workbooks can stall when series formulas span many rows. After that performance issue was fixed, Office Markdown still replaced native charts with their extracted HTML data tables, so the visual chart was lost.

Modification

• Read each one-dimensional chart formula range with a single iter_rows() stream and trim only trailing empty values.
• Parse chart titles, axis titles and scales, series colors, line widths, and dash styles from OOXML.
• Render supported scatter and bar charts from cached OOXML data as SVG images.
• Carry the SVG through the Office middle JSON image pipeline while preserving extracted HTML chart data.
• Emit the chart image first in multimodal Markdown and keep structured chart data in a collapsible chart content details block.
• Add regression coverage for streaming workbook reads, SVG visual properties, rotated value-axis labels, and Markdown image priority.

BC-breaking

No. Structured chart content remains available in middle JSON and content-list output. Multimodal Markdown now shows the chart image first and retains the HTML data table in a collapsible details block.

Testing

• Targeted Pytest: 5 passed.
• Ruff passes for all changed source and test files, excluding the module's existing ANN baseline.
• A 25-slide regression PPTX containing 16 charts completes Office parsing in about 3 seconds and produces SVG image paths for all 16 chart blocks.
• Slide 24 produces exactly two SVG charts. Headless-browser screenshots verified curve shape, legends, axis ranges and titles, colors, and solid/dashed line styles against the source slide.

Checklist

• Lint checks pass for changed code, excluding the module's existing ANN baseline.
• The performance bug and visual-output bug are covered by unit tests.
• The modification is covered by a real-document regression test and visual screenshot inspection.
• No new runtime dependency is required; reportlab is already declared by the project.

[github-actions]

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[xiejinwen113]

I have read the CLA Document and I hereby sign the CLA