Support latest Plurimath UnitsML build with generated Opal payload

[suleman-uzair]

Summary

This PR updates plurimath-js to work with the latest UnitsML stack by generating a UnitsML payload for Opal builds, updating the vendored Plurimath revision, and adding focused coverage for UnitsML conversions.

Changes

• generate a Ruby payload file for UnitsML database data during the build
• prepare the build to compile against the generated UnitsML payload input
• update vendor/plurimath to the required upstream revision
• add JS coverage for UnitsML AsciiMath conversions, including:
  • simple units like kg
  • prefixed units like MHz
  • compound units like g/mol
  • value + unit expressions like 9.8 "unitsml(m/s^2)"
• add Ruby coverage for the payload generator
• switch unitsdb, unitsml, and lutaml-model to branch-based git dependencies so CI validates the same dependency stack as the upstream changes

Upstream PRs

This PR should be reviewed together with:

• vendor/plurimath
  • Refactor MathML Parsing, Translation, and Opal Loading plurimath#412
• unitsml-ruby
  • Update UnitsML for latest MathML, Plurimath, and UnitsDB integration changes unitsml/unitsml-ruby#61
• unitsdb-ruby
  • Introduce context-first configuration and register Unitsdb model classes unitsml/unitsdb-ruby#25
• lutaml-model
  • [WIP] Fix Opal runtime compatibility for adapter loading lutaml/lutaml-model#649

Validation

• verified spec/unitsml.spec.js passes in the Podman environment

TODO Before Merge

• update vendor/plurimath to reference the commit from main after Refactor MathML Parsing, Translation, and Opal Loading plurimath#412 is merged, since the current branch reference is temporary

Notes

• Gemfile points to the corresponding upstream branches for CI verification
• this PR is intended to be validated together with the upstream dependency PRs listed above