fix: clarify frontmatter path resolution

[Cu19aaa]

Summary

• Add resolve_reference_path() for resolving OKF frontmatter references consistently.
• Keep absolute URLs unchanged and resolve bundle-relative paths safely.
• Prevent relative references from escaping the bundle root.
• Add tests covering URLs, bundle-relative paths, nested paths, and escape attempts.

Testing

• python -m pytest -q
• 47 tests passed
• git diff --cached --check passed

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