Put README's usage first, and the full reference after it

Usage: As a Library and Usage: As a Tool come back as short examples.
The tables follow under References.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PpWJK2RMkVoYYAJMFZMrDD

Share the listener counter with extension_test.go for declscope

65f9794 used engineTestListeners from the extension namespace, which
declscope refuses, and was pushed with that failure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017SjyfxMuyeVy5qq2AJHeyb