Phase 1: native charts page — directives replace the matplotlib cells

[mmcky]

Part of #28 (Phase 1). Gated on the report theme's Phase 1b exit.

Problem. lectures/charts.md at a92f911b draws five charts with matplotlib in executed code cells: the series-by-category heatmap, the most-violated rules, the change since the previous pass, the priority distribution by series, and the corpus-weighted category averages. Execution is the only reason the build needs a Python kernel, and the design brief specifies native equivalents (§5.8) that the report theme renders from the same CSVs.

Proposed work.

• Replace each code cell with its directive: {qe-heatmap}, {qe-rule-reach-chart} (limit 18, proposed rules marked), {qe-trend}, {qe-priority-bar} for all series, and a category-averages chart from the generic bar-list or stacked-bar primitive with the 8.6 and 5.0 threshold lines.
• Keep the prose introductions; drop the kernelspec and jupytext frontmatter once no page executes.
• Remove the execution configuration from the build and requirements.txt entries used only for charts, after checking nothing else executes.

Acceptance criteria.

• charts.md contains no code cells; myst build needs no kernel
• Each chart shows the same values as the matplotlib version for the current pass, checked against the CSVs
• The trend chart still degrades gracefully when only one period is on record