[css-color-hdr][editorial] Add Jzazbz prim-sec diagram and its generator script

Adds images/2020-prim-sec-JzCzHz.svg (rec2020 primaries/secondaries
plotted on the Jzazbz az,bz plane, with sRGB shown as a dashed
comparison overlay and a Jz/Cz/hz legend table below the plot) and
workings/JzCzhz-diagram/, a small script to regenerate it if needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: svgeesus

css-color-hdr-1/images/2020-prim-sec-JzCzHz.svg

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+# Jzazbz primaries/secondaries diagram generator
+
+Regenerates `css-color-hdr-1/images/2020-prim-sec-JzCzHz.svg`: a diagram
+plotting the rec2020 primaries and secondaries on the Jzazbz az,bz plane,
+with the sRGB gamut shown as a dashed comparison overlay and a Jz/Cz/hz
+legend table below the plot. (This is the sibling of
+`css-color-4/workings/oklch-prim-sec/`, which does the same thing for
+Oklab, but Jzazbz isn't in colorjs.io so the source Jz/Cz/hz and az/bz
+values are hardcoded in the script instead of computed live.)
+
+## Usage
+
+```
+node generate.mjs
+```
+
+No dependencies — the Jz/Cz/hz and az,bz values were computed once (see
+`../../jzazbz.js` for the conversion formulas) and are hardcoded at the
+top of `generate.mjs`. This overwrites `../../images/2020-prim-sec-JzCzHz.svg`.
+Diff/review before committing.
+
+## Notes
+
+- If the underlying Jz/Cz/hz values ever need recomputing (e.g. a change
+  to the Jzazbz conversion), redo the conversion for rec2020 and sRGB's
+  red/green/blue/cyan/magenta/yellow and update the `rec2020` / `srgb`
+  tables in `generate.mjs`; the plot framing, legend layout, and axis
+  labels are all generated automatically from those values.
+- If the diagram's aspect ratio changes, update the `<img width height>`
+  values wherever this file is referenced (currently not referenced from
+  `Overview.bs`).