chore: Switch to using symbols in FDv2 for consistency with v1

[jsonbailey]

Note

Unifies key handling to symbols across FDv2

• Normalize keys to Symbol in Change, ChangeSetBuilder#add_put/#add_delete, Protocol V2 (PutObject.from_h, DeleteObject.from_h), polling parsers (polling_payload_to_changeset, fdv1_polling_payload_to_changeset), and TestData V2 emitters
• InMemoryFeatureStoreV2#get now accepts string/symbol lookups via key.to_sym; store/data sorting and dependency tracking updated to operate on symbol keys
• Store writes/reads (changes_to_store_data, reset/apply, persistence paths) and FeatureStore interface docs updated to use symbol inner keys; FeatureStore#get docs accept String, Symbol
• TestDataV2 and its data source convert keys to symbols on update for flags/segments
• Extensive spec updates/additions validating key compatibility, polling/streaming behavior, FDv1 fallback paths, and persistent store outage recovery/flush in read-write vs read-only modes

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