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

[beekld]

Summary

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

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Note

Medium Risk
Changes the FDv2 source result contract and how sync responses are interpreted ahead of orchestrator fallback logic; mistakes could mis-signal FDv1 downgrade, but behavior is not yet wired to switching data modes.

Overview
FDv2 sources now surface a server-driven FDv1 fallback signal on every FDv2SourceResult. The fdv1_fallback flag moves from nested variant payloads to a single top-level field (default false), reflecting that it comes from the HTTP response, not the result kind.

Polling reads X-LD-FD-Fallback (value true, case-insensitive) from poll HTTP headers and sets fdv1_fallback on 304/200 outcomes and HTTP error paths; transport-level failures without a response still leave it false.

Streaming registers an SSE on_response hook, keeps latest_fdv1_fallback_ from the most recent response headers (cleared when a later response omits the header), and applies it in Notify to all delivered results—including errors after a directive.

Tests were updated for the new shape; new coverage exercises polling and streaming propagation. Orchestrator behavior is unchanged in this PR—the flag is stamped only for a follow-up to consume.

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