[M5] akb_adapter: emit title and source as node properties

[Korede-TA]

Goal

Populate properties.title and properties.source on document nodes so that deriveLabel() in the frontend can show meaningful names instead of raw IDs.

What to do

• In akb_adapter.py, when building document Node objects:
  • Add properties["title"] = CategoricalProperty(value=block.title) if block.title is set
  • Add properties["source"] = CategoricalProperty(value=block.source) if block.source is set
• For chunk nodes: optionally emit properties["position_label"] as "{chunk_index} / {total_chunks}" (useful in breadcrumbs)
• Update test_adapter.py to assert these properties are present on document nodes

Acceptance criteria

• Exported manifests have properties.title on document nodes
• deriveLabel() in frontend shows document title, not raw block ID
• test_adapter.py covers this