Add NeedleEvent interface as the canonical internal shape versioned at
schema-version 1. Parser validates wire format and asserts schema version
on incoming events. Legacy LogEvent retained as backward-compatible adapter.
docs/schema.md documents all fields, the (worker_id, sequence) ordering
contract, and the full event taxonomy cross-referenced with NeedleEventType.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
