feat: Add the FDv2 polling data source

[beekld]

Summary

Adds the first concrete FDv2 sources: a polling initializer and synchronizer, both built on the existing HttpTransport.

A few behaviors worth noting:

• The synchronizer clamps its poll interval to a 30-second minimum.
• Requests carry the standard authentication, application tag, and instance-id headers, and send the current selector as the request basis.
• Recoverable HTTP statuses surface as interruptions and unrecoverable ones as terminal errors.

The source also reports an FDv1 fallback directive when a response signals one, which the orchestrator will act on in later work.

NOTE: This PR adds a direct dependency on percent-encoding, but this is low risk, because we already depend on that crate indirectly through other dependencies.