feat: propagate FDv1 fallback directive from FDv2 polling and streaming sources

[beekld]

Summary

Stacked on #537. Each FDv2 source now inspects the X-LD-FD-Fallback response header (value "true", case-insensitive) and stamps FDv2SourceResult.fdv1_fallback on every emitted result. The orchestrator does not yet consume the flag — that comes in a follow-up.

Design notes

• fdv1_fallback is hoisted from per-variant (was on ChangeSet, Interrupted, TerminalError, Goodbye) to a top-level field on FDv2SourceResult. The flag is a property of the HTTP response, not of the result kind. Touches every existing construction site (mechanical: drop trailing false arg, the top-level default does the rest).
• Streaming synchronizer holds a latest_fdv1_fallback_ bool on State. Each OnResponse overwrites it from the incoming headers, so a fresh response without the directive clears it. Events between two responses share the flag from the most recent response.

Test plan

• New fdv2_polling_impl_test.cpp covers propagation across all variants + value comparison
• New streaming tests cover wiring + clearing on subsequent response
• Full server + SSE suites green