refactor: Remove the test-only Shutdown source result

[beekld]

Summary

FDv2SourceResult::Shutdown was produced only by the orchestrator's unit tests, which used it as a shortcut to stop a run. No real data source emits it. In production a run ends through the broadcast shutdown channel or by exhausting every source.

This removes the variant and reworks the affected tests to end through the real shutdown channel, so they exercise how the orchestrator actually terminates rather than a path that cannot occur. The diff is large but mostly mechanical. The bulk is updating the orchestrator test call sites to the reworked mock.

This also fixes CI on #193, whose narrowing of the FDv2 dead-code suppression exposes the variant this removes.

---

Note

Overview
Removes FDv2SourceResult::Shutdown and the orchestrator branch that treated it as an early exit. Production runs already stop via the broadcast shutdown channel or when every synchronizer is blocked.

FDv2 data-system unit tests are reworked so scripted mocks end runs the same way: MockSynchronizer swaps the old hang flag for an optional shutdown sender (signal when the script ends, otherwise idle for timer tests), sync_factory folds in the former fdv1_factory via an is_fdv1_fallback flag, and exhausted initializer scripts default to Interrupted instead of Shutdown.

Most of the diff is mechanical test call-site updates; runtime behavior for real sources is unchanged aside from dropping an unreachable enum variant.

^{Reviewed by Cursor Bugbot for commit 8ff2c82. Bugbot is set up for automated code reviews on this repo. Configure here.}