feat: Opal boot file and boot-file verification

[ronaldtse]

Summary

Adds the canonical Opal entry point for unitsdb and a CI check that verifies it stays well-formed.

• Adds lib/unitsdb/opal.rb: a single eager-require file that consumers (unitsml-ruby, plurimath-js) pass to Opal via -r unitsdb/opal. Under Opal, Ruby's autoload does not lazy-execute, so consumers need this canonical entry point rather than hand-listing files.
• Adds spec/unitsdb/unitsdb_opal_boot_spec.rb: a static MRI-run spec that verifies:
  • The boot file eager-requires every unitsdb/*.rb file that lib/unitsdb.rb exposes.
  • The boot file does not leak Opal-gated entry points (cli, commands).
  • The boot file only references files that exist on disk.
  • The boot file actually compiles under Opal::Builder with the native-only lutaml/model dependency stubbed (the gem's consumer is responsible for providing a working lutaml-model bundle at runtime).
• Adds .github/workflows/opal.yml to run the spec on every push and pull_request.
• Adds opal (~> 1.8) to the Gemfile as a development dependency for the compile check.

Scope boundary

This PR establishes the contract between unitsdb-ruby and its Opal consumers — a single entry point and a CI check that it is well-formed. It does not make unitsdb-ruby itself run under Opal at runtime, because:

• unitsdb-ruby uses Lutaml::Model::Serializable for every model class, so runtime Opal execution requires lutaml-model to work under Opal (separate concern, owned by lutaml-model and plurimath-js's bundle).
• The boot file's job is to be compilable by Opal. The runtime bundle is the consumer's responsibility — plurimath-js already does this at bundle time via the boot file added here.

Test plan

• bundle exec rspec spec/unitsdb/unitsdb_opal_boot_spec.rb — 4 examples, 0 failures locally.
• bundle exec rake — 810 examples, 0 failures (no regression).
• bundle exec rubocop — clean.
• CI green on this PR.