ci: dispatch to unitsml/unitsdb-js on every unitsdb release

[ronaldtse]

Summary

Adds notify-unitsdb-js.yml, a cross-repo trigger that fires
repository_dispatch (do-release) to
unitsml/unitsdb-js every
time lib/unitsdb/version.rb changes on main.

unitsml/unitsdb-js is a new release-only repo that publishes the
Opal-compiled unitsdb-ruby bundle to npm as @unitsml/unitsdb. It
was created in conjunction with this change.

What happens end-to-end

1. release.yml (existing Cimas chain) bumps lib/unitsdb/version.rb
   and tags vX.Y.Z on main.
2. This workflow sees the version bump, extracts X.Y.Z, and
   dispatches do-release to unitsml/unitsdb-js with payload
   { version, ruby_ref }.
3. unitsdb-js's release.yml checks out unitsdb-ruby at
   ruby_ref, runs Opal, dumps the bundled database to JSON, wraps
   with an esbuild-built TypeScript façade, and publishes
   @unitsml/unitsdb@X.Y.Z to npm.

Why this shape

• Separate repo: keeps Ruby and JS tooling/CI/contributors
  cleanly separated; the Ruby repo stays the single source of truth.
• Trigger via repository_dispatch: matches the gated-chain
  pattern the Cimas release already uses for downstream consumers.
• Idempotency: unitsdb-js's workflow checks
  npm view @unitsml/unitsdb@VERSION before publishing and skips
  cleanly if the version is already live. Re-dispatching after a
  transient failure just works.

Required secrets

• UNITSML_CI_PAT_TOKEN (already configured) — used by
  peter-evans/repository-dispatch@v3 for the cross-repo dispatch.
• NPM_TOKEN — must be set on unitsml/unitsdb-js for the publish
  step. This is not yet in place; please add it before the next
  unitsdb release goes out, otherwise the JS dispatch will fail at
  the publish step.

Test plan

• CI on this PR passes (workflow file is valid YAML).
• After merge, the next unitsdb patch release will trigger
  unitsml/unitsdb-js's release.yml. Verify
  @unitsml/unitsdb@<version> appears on npm.
• Re-triggering the workflow (via workflow_dispatch on
  unitsdb-js) with an already-published version exits cleanly
  with the skip warning.