feat: orchestrator switches to FDv1 fallback on directive

[beekld]

Summary

Implements the FDv1 fallback directive in the FDv2 data system with TTL-based recovery.

• FDv2SourceResult carries std::optional<FDv1FallbackDirective> with a TTL (default 1h, 0 = indefinite).
• Sources parse X-LD-FD-Fallback-TTL (header) and protocolFallbackTTL (goodbye).
• On directive: switch to FDv1 if configured, otherwise disconnect (kInterrupted). Schedule retry after TTL.
• On TTL elapse: SourceManager::SwitchBackToFDv2() re-enables FDv2 (including previously terminal-error-blocked factories), blocks FDv1, restarts synchronizers.

End-to-end fallback will work once an FDv1 streaming source is wrapped as an IFDv2Synchronizer (#540).

Test plan

• SourceManager tests cover the new block/unblock semantics
• Source-level tests cover TTL parsing (HTTP + goodbye), default, malformed
• Orchestrator tests cover directive flows in initializer/synchronizer paths, TTL elapse rebuild, FDv2 recovery
• Full server suite green

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Fallback goodbye still restarts SSE

Medium Severity

When a goodbye event includes an FDv1 fallback directive (protocolFallbackTTL or a merged fallback header on the result), the orchestrator now switches away from the FDv2 streaming synchronizer and shuts it down. The goodbye handler still always calls async_restart afterward, so the client can start a reconnect on the same connection the orchestrator is closing.

 

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