feat: Update ChangeSet to always require a Selector

[jsonbailey]

Note

Medium Risk
This is a type/contract change that affects how FDv2 updates are applied and when data is considered ready/persistable; downstream code assuming selector can be None may break or change behavior.

Overview
ChangeSet now always carries a Selector (non-optional). This removes None handling throughout the FDv2 path and standardizes on Selector.no_selector() for “no selector” cases.

Initializer/store logic is simplified accordingly: persistence and readiness gating now use change_set.selector.is_defined() directly (no selector is not None checks), and Store._set_basis/_apply_delta signatures are tightened to require a Selector. Tests are updated to reflect the new invariant (selectors are always present; assertions now check is_defined() instead of is not None).

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