Make BinaryStorage thread-safe

[imurashka]

The package described itself as "thread-safe", but the current implementation has no thread-safety guarantees. The misleading wording has been removed from the package description for now; this issue tracks actually making the storage safe for concurrent use.

Current state

There is no synchronization anywhere in the runtime:

• No lock / Monitor / Interlocked / ReaderWriterLock, no System.Collections.Concurrent, no volatile.
• State is held in plain Dictionary<string, Record> (src/Runtime/BinaryStorage.cs), which is not safe for concurrent reads/writes.
• Get / Set / Remove, the reactive collections, the dirty flag and the change-scope counter are all mutated without guarding.

What does exist (and is unrelated to thread-safety) is atomic disk writes: data is written to a temp file and then swapped in via delete + move (src/Runtime/BinaryStorageIO.cs), which protects the file from corruption on an interrupted save.

Goal

Make concurrent access to a single BinaryStorage instance safe, or explicitly define and document the supported threading model.

Things to decide / cover

• Locking strategy (single lock vs reader/writer) and its scope across Get/Set/Remove/Save/MultipleChangeScope.
• Thread-safety of the reactive collections returned by GetListOf / GetSetOf / GetDictionaryOf (they mutate the parent storage on change).
• Behavior of Save / auto-save when changes happen concurrently.
• Whether change events (OnKeyAdded/OnKeyChanged/OnKeyRemoved) are raised on the caller thread or marshalled.
• Tests covering concurrent access.
• Restore the "thread-safe" wording in the package description once guarantees actually hold.