feat: chart cells, header sub field, responsive chart column sizing

[tmustier]

Summary

Adds chart cell support — embed native PowerPoint bar charts directly in table cells via YAML references. Also introduces the sub field for semantic header subtitles and fixes column sizing for chart-heavy tables.

Chart Cells

Table cells can now reference chart data points using chartname-N syntax:

charts:
  revenue:
    dir: vertical
    values: [100, 200, 300]
    format: "{:.0f}"
    color: "#0D193B"

table:
  cells:
    - [revenue-1, revenue-2, revenue-3]

Each chart reference renders as a native <c:chartSpace> shape sized to the cell. Supports:

• Vertical and horizontal bar charts
• Scale groups — charts sharing a scale group use the same axis max for visual comparability
• Data labels — positioned outside bars with per-label x-offsets (horizontal) or headroom (vertical)
• Format strings — Python format specs (e.g. "{:.1f}x", "€{}m") converted to Excel number formats
• Charts delegate to an external generator module (CLEAN_SLIDES_CHARTS_PATH env var or charts.py in the project)

Header sub Field

New optional sub field on all header types for units, qualifiers, and annotations:

col_superheaders:
  - { label: "Net Debt / EBITDAaL", sub: "Ratio", span: 2 }

col_headers:
  - { text: "Interest paid", sub: "€m" }
  - "FY26E"
  - { text: "IRR", sub: "%" }

row_header_col_header: { text: "Net debt", sub: "€m" }

row_headers:
  - { text: "€6,000m", sub: "Base Case" }

The sub text renders as a soft line break (<a:br/>) within the same paragraph — non-bold, body text color, no extra paragraph spacing. Plain strings still work unchanged.

Column Sizing Fix

Chart-only columns no longer claim an equal share of the area as their max width. They use their header text width instead, expanding proportionally with remaining slack. This prevents text-only columns from being squeezed when many chart columns are present.

Changes

• clean_slides/spec.py — ChartDef, ChartRef dataclasses; sub field on ColSuperHeader and TableSpec headers; rich content parsing for headers
• clean_slides/chart_render.py — new module: chart shape generation, label positioning, scale groups
• clean_slides/charts.py — chart module loader
• clean_slides/content.py — header_text_and_sub() helper; _make_sub_paragraph(); sub on Paragraph
• clean_slides/renderer.py — chart cell rendering; use_line_breaks=True for header cells
• clean_slides/sizing.py — chart row/column equalization; sub-aware height/width calculations; responsive chart column sizing
• clean_slides/verification.py — pass raw header values (not stringified) to overflow checks
• Tests added for chart cell parsing and rendering

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