feat: Update ChangeSet to always require a Selector

[jsonbailey]

Note

Medium Risk
This is a cross-cutting type/contract change affecting data delivery and store initialization paths; mistakes could impact persistence decisions or readiness signaling even though the behavioral change is small and well-covered by tests.

Overview
ChangeSet.selector is now non-optional, standardizing on always carrying a Selector (using Selector.no_selector() when absent) and removing None checks across FDv2 code paths.

Initializer/polling and FDv2 readiness/persistence logic now keys solely off selector.is_defined() (e.g., when deciding Basis.persist and when to set the ready event), and store internals (_set_basis/_apply_delta) accept a required Selector. Tests were updated accordingly to assert selector.is_defined() rather than non-nullness.

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