Add Opal database payload loader

[suleman-uzair]

Adds an explicit Unitsml::Unitsdb::Database.load_opal_payload hook for Opal builds.

This lets downstream packages provide the bundled UnitsDB payload without generating subclasses or reimplementing from_db. The Opal loader remains owned by unitsml-ruby, including the missing-payload error path.

[Copilot]

Pull request overview

Adds a dedicated Opal-only hook to inject a bundled UnitsDB payload at runtime, allowing Opal consumers to use Unitsml::Unitsdb::Database.from_db without subclassing or re-implementing loading behavior.

Changes:

• Add Unitsml::Unitsdb::Database.load_opal_payload and opal_payload helpers to support injected payloads (with fallback to the DATABASE constant).
• Update Opal from_db path to use the new opal_payload accessor and raise when missing.
• Extend specs to cover the new Opal payload injection behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/unitsml/unitsdb/database.rb	Introduces load_opal_payload/opal_payload and switches Opal loading to use the payload accessor.
spec/unitsml/unitsdb/database_spec.rb	Resets Opal payload between examples and adds coverage for the new payload injection hook.

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