* M2.1: preserve chart parts through native saves
Serialize retained chart relationships and payloads in native files.
Add native and headless file-command round-trip regressions.
Contribution on behalf of @MMexicann.
* Format: bound opaque chart parts as native ZIP entries
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
* Model: track transient native chart part entries
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
* PPTX: initialize native chart entry metadata
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
* Engine: adapt chart native roundtrip fixtures
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
* Docs: describe bounded chart sidecars and validation
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
* Roadmap: record bounded chart persistence evidence
Address maintainer feedback on the existing PR. Preserve upstream work and document tested scope and validation limits.
---------
Co-authored-by: skylar taylor-barrick <hi@skylar.cc>
