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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Note

Medium Risk
Changes core flag-data orchestration and protocol fallback behavior; mis-handling could leave clients on the wrong protocol or retry timing, though behavior is heavily tested.

Overview
Adds server-directed FDv1 fallback for the FDv2 data path: results now carry an optional FDv1FallbackDirective (TTL, default 1h; 0 = stay on FDv1 indefinitely) instead of a boolean flag.

Parsing: Goodbye JSON gains protocolFallbackTTL; polling and streaming read X-LD-FD-Fallback / X-LD-FD-Fallback-TTL, with body/goodbye directives overriding headers.

Orchestration: On directive, SourceManager blocks FDv2 factories and selects an IsFDv1Fallback() synchronizer; missing adapter → kInterrupted without forcing kOff. A cancellable timer calls SwitchBackToFDv2() (re-enabling FDv2 factories blocked for terminal errors) and restarts synchronizers. Destructor/Close() no longer sets status to kOff; retry timers are cancelled on close.

Other: Logging tweaks (initializer/synchronizer start, deduped synchronizer interrupts). Broad unit/orchestrator tests cover TTL parsing, switching, recovery, and edge cases.

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