Validate discrete chartProperties values; docs show accepted values not labels

[Copilot]

Following the generated Vega-Lite reference docs for the Regression template rendered a blank chart: the docs' Domain column listed option labels (Logarithmic) instead of accepted values (log), and the assembler copied the unrecognized value verbatim into the Vega-Lite regression transform, which the renderer silently rejects.

assembleVegaLite({
  /* … */
  chart_spec: { chartType: "Regression",
    encodings: { x: { field: "flops" }, y: { field: "loss" } },
    chartProperties: { regressionMethod: "Logarithmic" } },  // label copied from docs
});
// before → "method": "Logarithmic"  (invalid; blank chart)
// after  → "method": "log"          (+ info warning about the coercion)

Changes

• core/normalize-properties.ts (new) — validates discrete chartProperties against the template's declared options before instantiate:
  • valid value → passthrough
  • known label (case-insensitive) → mapped to its value, info warning
  • unrecognized value → dropped so the property falls back to its default, warning (preserves the "safe to pass" contract instead of emitting an invalid backend spec)
• Assemblers — wired the normalization into all three backends (Vega-Lite, ECharts, Chart.js), so the class of bug can't recur per-backend.
• scripts/gen-chart-reference.ts — discrete Domain/Default columns now render the accepted value in code font with the label as secondary text (e.g. `log` (Logarithmic)); options with an undefined value render as Label _(default)_. Regenerated docs/reference-*.md.
• Tests — tests/chart-properties-validation.test.ts covers valid value, label coercion, case-insensitivity, invalid-value drop, and default.

Note for reviewers

An unrecognized value is dropped-to-default with a warning rather than throwing, matching the docs' "non-applicable parameters are safe to pass" wording. Switching to a hard error is a one-line change in normalize-properties.ts if preferred.