fix: add end-to-end flush semantics for flush(cb)

[mcollina]

Summary

Fixes #197 by making ThreadStream#flush(cb) wait for end-to-end worker destination flush, not only shared-buffer drain.

Changes

• Added cross-thread flush protocol:
  • main thread sends { code: 'FLUSH', context: 'thread-stream', id }
  • worker replies { code: 'FLUSHED', context: 'thread-stream', id }
• Added context validation on both sides to prevent message collisions with other libraries using worker message channels.
• flush(cb) now:
  1. waits for local/shared buffer drain
  2. waits for worker flush acknowledgment
• Added tracking for pending flush callbacks and failure propagation on worker exit/destroy.
• Worker flush behavior now supports:
  • destination.flush(cb) (including promise/sync variants)
  • fallback to destination.flushSync()
  • fallback wait for drain when backpressured
  • immediate ack when no flush primitive is available
• Added tests for all requested flush semantics and worker-exit failure path.
• Added missing flush(cb?) declaration in index.d.ts.
• Updated README to describe stronger async flush semantics.

Validation

• npm run lint
• npm run test:ci
• npm run build

Fixes #197