feat: Honor the FDv1 fallback directive in the orchestrator

[beekld]

Summary

An initializer or synchronizer can report an FDv1 fallback directive alongside its result. When the orchestrator sees one, it blocks the FDv2 sources and switches to the FDv1 fallback synchronizer. A directive takes precedence over a terminal error reported by the same source.

The directive carries a TTL, which the orchestrator honors:

• A non-zero TTL schedules a retry that returns to FDv2 once it elapses.
• A zero TTL falls back indefinitely.

The FDv2 spec describes the fallback as terminal, but the SDK follows the TTL instead. The retry runs on an independent timer, so it still fires when no synchronizer is available. Without that, an FDv1 fallback that itself hit a terminal error would strand the data system.