Improve notifications, batching and non-blocking views

[ChrisPulman]

Multiple changes to notification semantics, batching and view construction:

• Implement INotifyPropertyChanged across QuaternaryBase, ReactiveList and ReactiveGroup and cache common PropertyChanged/NotifyCollectionChanged EventArgs to reduce allocations and ensure Count/Item[] notifications on mutations.
• Refactor batching: introduce CreateBatch helpers, return PooledBatch with a ReturnToPool flag, and update usages to avoid returning pooled arrays containing live references. PooledBatch now conditionally returns arrays and uses ArrayPoolClearHelper when clearing.
• Optimize ReactiveList internals: use array copies / AddRange and ReplaceAll for observable backing collections, introduce GetMultisetDifference for Edit diffs, add SetItem helper, use an integer _skipInternalNotifications counter to coordinate internal vs external stream notifications, and raise batched collection/INPC events with fewer allocations.
• QuaternaryList: coalesce removed items by shard and emit a single batch via EmitRemovedItems, avoiding duplicate index notifications and preserving order.
• Views: make DynamicReactiveView, DynamicSecondaryIndexReactiveView and DynamicSecondaryIndexDictionaryReactiveView non-blocking at construction by using TryGetLatest to capture an initial filter/keys without blocking on FirstAsync, and adapt subscription handling accordingly.
• ReactiveListExtensions: make ToChangeSets emit an initial ChangeSet for preloaded sources; improve batch filtering to allocate exact-sized arrays and return non-pooled batches.
• Core: add CP.Reactive.Internal usage and minor namespace/using adjustments.
• Tests & benchmarks: add DynamicDataParityBenchmarks and ReactiveListNotificationComplianceTests; add a test asserting Connect emits an initial snapshot; adjust existing tests to ignore the initial Connect snapshot where appropriate.

These changes reduce allocation churn, avoid blocking on subjects at construction, and improve compliance with INPC/INCC contracts while simplifying batch lifecycle handling.