feat(resolver): optional merged properties from multiple matching sources

[xoolive]

Problem

Notebook TODO requests merged metadata across all sources, not only first-hit source.

Proposal

Add optional mode for resolve:

• default: first-hit behavior (current)
• optional: merge properties from all matching sources

API sketch

resolve(query, { merge: true })

Merge output

• properties.source: primary source
• properties.sources: array of contributing sources
• merged fields with deterministic precedence

Acceptance criteria

• Backward-compatible default behavior.
• Merge mode tested on airport + navaid examples.