Add isLocked() to Storage and EventStore

[Copilot]

There was no public API for clients to check whether a storage is currently locked by a writer — particularly useful for read-only consumers who want to know if a writer is active without attempting to acquire the lock themselves.

Changes

• ReadableStorage: moves lockFile path computation into the base class constructor (was only in WritableStorage); adds isLocked() returning fs.existsSync(this.lockFile).
• WritableStorage: removes now-redundant this.lockFile assignment (inherited); lock() now uses isLocked() as the primary pre-check before mkdirSync, with the EEXIST catch retained for the race-condition edge case.
• EventStore: adds isLocked() delegating to this.storage.isLocked().

Usage

const writer = new EventStore({ storageDirectory });
const reader = new EventStore({ storageDirectory, readOnly: true });

reader.isLocked(); // true — writer is holding the lock
writer.close();
reader.isLocked(); // false

Likewise available directly on Storage and ReadOnlyStorage:

const reader = new Storage.ReadOnly({ dataDirectory });
reader.open();
reader.isLocked(); // true if a WritableStorage has the lock, false otherwise