Graph discovery: index normalized properties and extend find()

[toddysm]

Part of #168 (gap analysis bucket 3a: discovery).

Problem

queries.find() and the /search endpoint only filter by annotation name=value, artifact/media type, or repository. #168 requires discovery by indexed property/value, related packages, files, vulnerabilities, signers, and verification state.

Scope

• Index normalized properties for efficient filtering while preserving the original annotation key/value.
• Extend find (CLI + graph-service /search) to filter by: indexed property/value; package; file path/digest; CVE; signer; verification state.
• Return unambiguous results across multiple repositories/roles for a digest.

Acceptance criteria

• Normalized property index added without requiring a schema change per key.
• find//search support the new filters with tests.
• Docs updated.

Depends on the inventory work (layers/packages/files) and typed-evidence work for package/file/CVE/signer filters.