Emit all theme tokens, fix callout colours, load Inter

Three tokens the callouts named had never been declared, and an undefined
custom property does not fall back — it invalidates the whole declaration, so
`border-color` reset to `currentColor` and three of the five callout types drew
their bar in the text colour. They now point at the tokens the MDXEditor
admonition variables above them already claimed.

`--color-success` then still failed, for a different reason: Tailwind emits a
theme variable only where it can see a utility class using it, and it looks for
those in the content files rather than the stylesheets. A token read solely
through `var()` in hand-written CSS was declared and pruned. `--color-warning`
beside it worked purely because a `text-warning` class happened to exist
somewhere. `@theme static` emits the palette whole.

InterVariable was named in two places and installed in none, so the editor had
been falling through to the system font since it was written. The stack is now
declared once, as a token the editor reads, and the font is loaded — the
optical-sizing cut, which retunes the letterforms across the range the type
scale spans. Roman and italic both: italic is a separate face rather than a
slant, and a plan leans on emphasis. Greyscale smoothing comes with it, since
macOS subpixel rendering thickens Inter past the weight asked for.

`scripts/check-tokens.ts` runs in CI and fails on any `var()` that cannot
resolve, and separately on an `@theme` that loses `static` — which would
silently invalidate the first check. It reads sources only; a built `dist`
carries third-party CSS naming tokens we neither wrote nor can define.

Also adds `--color-surface`, so the rails and header recede and the plan is the
only white surface on the page.

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