fix: handle annotation availability and external labels, reporting only available cell annotations

[Sergio-ote]

What & why

Improved annotation handling so annotation-dependent plots, composition outputs, and report panels are generated only for annotation sources containing meaningful labels. Previously, when no manual cluster-to-cell-type TSV mapping was provided, an all Unannotated cell_type_tsv placeholder could be treated as a valid annotation. The placeholder is still retained in the h5ad for compatibility but is excluded from annotation outputs.

External annotations now support arbitrary source-column names from metadata TSVs or decoupled h5ad objects, with consistent handling of retained unannotated cells.

Validation

• Workflow lint passes against .test/
• Decoupled .test/ dry-run builds the CoreBlock DAG
• Affected HeadBlock mode was dry-run with a complete mode-specific config and representative inputs, if applicable
• Config validates against workflow/schemas/config.schema.yaml
• mkdocs build --strict passes, if documentation was changed
• Catalogue rule graph was generated before a release, if applicable
• Parameter names match between .smk rules and scripts; documentation was updated if behaviour changed