refactor: add Apply method to MemoryStore for transactional change sets

[beekld]

This is a simple PR to add an equivalent of Java's InMemoryStore#apply. Unit tests are added. I don't predict anything controversial here.

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Note

Medium Risk
Introduces new mutation path that can clear and repopulate the in-memory flag/segment cache, so incorrect change-set composition or missed object types could cause stale or missing data. Scope is limited to MemoryStore and is covered by focused unit tests.

Overview
Adds MemoryStore::Apply(FDv2ChangeSet) to apply FDv2 transactional updates: no-op for kNone, merge for kPartial, and clear + reinitialize for kFull (also setting initialized_).

Includes a new memory_store_apply_test.cpp test suite validating initialization behavior, full-replace semantics, and partial update/preservation for flags and segments.

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