Skip reindexing when commit() creates a new write stream

[Copilot]

createEventStream always reindexes the full store when creating a new stream. For ad-hoc read streams this is necessary, but for write streams created implicitly by commit(), no existing event can ever match the brand-new stream name — reindexing is wasted work.

Changes

• WritableStorage.ensureIndex(name, matcher, reindex = true) — added reindex flag; when false, skips the forEachDocument scan and registers the new index empty.
• EventStore.createEventStream(streamName, matcher, reindex = true) — threads the flag through to ensureIndex. Public default remains true, preserving existing behaviour for ad-hoc read streams.
• EventStore.commit() — passes reindex: false when auto-creating a write stream, since uniqueness of the stream name guarantees zero historical matches.

// Ad-hoc read stream: reindexes to find all matching existing events (default)
eventstore.createEventStream('my-projection', e => e.payload.type === 'OrderPlaced');

// Write stream via commit(): new stream index starts empty, no scan needed
eventstore.commit('order-123', [{ type: 'OrderPlaced' }]);

[coveralls]

coverage: 98.154% (+0.03%) from 98.127% — copilot/add-toggle-to-create-stream-method into main