rdfexport metadata canonicalisation follow-up (2025-10-23)

Summary

Normalised DrawIO metadata handling around a canonical <gbadMetadata> container in TypeScript (src/rdfexport.ts) so parser settings, stripHtml, and strict flags persist across UI actions and serialised XML payloads.
Synced Python overrides to prefer the new tag while tolerating legacy <UserObject>/<object> containers (legacy/overrides/metadata_extraction.py, metadata_cleanup.py, and rml_export.py).
Taught the debugger metadata patcher (debug/__main__.py) and test fixtures (including tests/fixtures/Flowchart_tweaked.drawio) to surface prefixes via the canonical node, resolving the Flowchart regression and keeping regenerated .drawio assets consistent with the plugin.
Regenerated Bun + pytest debug logs and scenario artefacts so CI mirrors the updated metadata contract.

Verified _strip_metadata_user_object converts <gbadMetadata> back to the expected <mxCell id="0"> during parser regeneration.
Confirmed Flowchart_tweaked.drawio previously lost the kb prefix in TypeScript pipeline runs because the debugger still injected a legacy <UserObject> wrapper, overwriting metadata before export.
Added defensive conversions in the debugger so any existing metadata envelope is promoted to canonical form before overrides mutate it.