docs: propose auxiliary definitions

[dakusui]

Summary

• define the auxiliary namespace, the root $aux object whose auxiliary definitions are available during processing but absent from output
• specify composed scope, evaluation order, jq access, references, paths, escaping, and interaction with $local
• plan removal of yq++ underscore stripping and divide implementation into ordered review phases

This is the planning-only pull request for #108. It contains no implementation changes and does not close the issue.

Validation

• openspec validate --all --strict
• git diff --cached --check