Adds a global `projections` section to GET /metrics exposing each cursor's
position (last_event_id/last_offset), determinism checksum, and lagMs.
smoke-projection.mjs injects events and asserts the projection's ordered
high-water-mark advances with a 64-hex rolling checksum (KG-06).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
