feat: Add FDv2 source traits and result types

[beekld]

Summary

Defines the interfaces the FDv2 data sources implement, ahead of the concrete polling and streaming sources. This adds:

• Initializer and Synchronizer traits, whose methods return a boxed future so the traits stay object-safe.
• The FDv2SourceResult enum returned by those methods, plus its accompanying event and error types.

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Note

Overview
Introduces the FDv2 data-source contract that upcoming polling/streaming implementations will share.

Adds source under fdv2 with Initializer (run → boxed future) and Synchronizer (next(selector) → boxed future), both object-safe via BoxFuture. Each call yields an FDv2SourceEvent wrapping FDv2SourceResult (ChangeSet, recoverable Interrupted, TerminalError, Shutdown, Goodbye) plus optional FDv1FallbackDirective (TTL). Shared ErrorKind / ErrorInfo types classify failures for status reporting later.

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