Add two-stop shadow scale tokens

One stop has to choose between the contact shadow where a surface meets the
page and the ambient fall-off around it, and whichever it picks reads as a
blurred border. Three were written by hand at three call sites — 8% at 2px,
12% at 3px, 12% at 8px — which is three answers to a question nobody had asked
twice on purpose.

Mixed from `foreground` rather than black, since the page is warm-neutral and a
black shadow greys toward a different hue than the borders it runs into.

Declared under Tailwind's `--shadow-*` namespace, so the two `shadow-md` sites
in the toolbar and the slash menu pick the scale up without being touched.

`@theme` merges with Tailwind's theme rather than replacing it, so the rungs we
do not declare go on resolving to a different design system's — `shadow-xl`
hardcodes `#0000001a`, pure black, with no reference to the theme at all.
`--shadow-*: initial` deletes the namespace first, so reaching for a rung we do
not own draws nothing instead of something plausible. Needs malva 0.16 to parse,
hence the dprint bump.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>