Make InMemory event store thread-safe

[artnim]

Problem

InMemoryEventStore and InMemoryEventStream are not safe for concurrent use:

• InMemoryEventStore.CreateAsync does a check-then-add (ContainsKey + Add) on a plain Dictionary — concurrent creates corrupt the dictionary (InvalidOperationException: Operations that change non-concurrent collections must have exclusive access…), and once corrupted every subsequent store call fails. Two racing creates for the same id can also both succeed.
• InMemoryEventStream.AppendAsync computes Version + 1 by enumerating the live List and then Adds to it — concurrent appends lose events, assign duplicate versions, or corrupt the list.
• InMemoryTransactionalBatch.CommitAsync appends to the same raw list unguarded.

This bites in practice when the store backs an ASP.NET Core test host (WebApplicationFactory): background services or notification handlers append events on thread-pool threads while the test thread seeds streams. We hit it as a flaky CI failure in a downstream project — one corruption cascaded into three failing tests.

Fix

• InMemoryEventStore: ConcurrentDictionary + TryAdd, making creation atomic (exactly one winner, the rest get EventStreamAlreadyExistsException).
• InMemoryEventStream: a Lock guards all reads and writes of the event list; appends compute the version inside the lock, and the List* methods materialize a snapshot under the lock so enumeration never observes a mutating list.
• InMemoryTransactionalBatch: commits under the owning stream's lock, so batched version computation stays consistent with concurrent direct appends.

No public API changes; Lock is fine since the package targets net10.0.

Tests

First commit adds the failing tests (concurrent creates corrupt the store; concurrent appends lose events/duplicate versions — reproduced 3/3 runs before the fix), second commit makes them pass. A third test pins the exactly-one-winner contract for same-id concurrent creates. Full solution builds; Papst.EventStore.Tests, CodeGeneration.Tests, and the InMemory tests all pass (the Cosmos/MongoDB integration suites need their emulators and were not run locally).