v0.3.0 — CI in about two minutes; no change to the packages

The package code is the same as 0.1.0 and 0.2.0; this release changes how
sheetpane is checked and published. It is the first version meant to
reach npm from CI, through publish.yml with signed provenance.

npm i @sheetpane/grid@0.3.0

Before you upgrade from 0.1.0: sheetpane needs Node 22 or later
wherever it runs in Node (0.2.0's change). Browsers are unaffected.

Release and tooling

• CI runs its jobs in parallel: typecheck, coverage, the pack check
  and demos, the three-OS matrix, the templates, and the test suite in
  four shards balanced by measured time. The pnpm store and Chromium are
  cached.
• publish.yml asks npm about every package before uploading any. It
  prints npm's reason for each refusal, so a release is never left half on
  npm.
• Release titles say what the release is (v0.3.0 — …), taken from
  the CHANGELOG heading.
• A test helper that took 117 s now takes 0.3 s: the TSDoc check reads
  each declaration file once.
• Every GitHub Action moved to its current major.