chore: Remove dead code, clarify names, other cleanup

[jsonbailey]

Note

Medium Risk
Touches core polling/streaming and persistence decision logic; changes could affect whether data is persisted or when the client stops/reverts during fallback, though behavior is now more explicit and tests were added.

Overview
Improves FDv2 data-source behavior around selector handling, persistence, and fallback. ChangeSetBuilder.no_changes() now returns an explicit Selector.no_selector() (instead of None), and polling persistence is only enabled when the selector is defined—preventing FDv1 fallback payloads (which have no selector) from being written to persistent storage.

Cleans up endpoint constant naming (FDV1_POLLING_ENDPOINT, FDV2_POLLING_ENDPOINT), removes unused RepeatingTask wiring in PollingDataSource, and ensures streaming shuts down immediately when the server indicates revert_to_fdv1.

Includes small store/apply_delta cleanup, docstring typo fixes, and test updates/additions to cover FDv1 payload parsing and the new persistence/selector semantics.

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