HTTP gateway

Opt-in HTTP/1.1 surface for operators, browsers, and tools that prefer HTTP/JSON. Build with --features http-gateway; add ,tls for HTTPS.

Routes: /v1/sql, /v1/op, /v1/health, /v1/metrics (Prometheus text v0.0.4). Authorization is Bearer with constant-time comparison; exactly-once headers X-Kessel-Client-Id + X-Kessel-Req-Seq are both-or-neither. Full route table, status-code mapping, Prometheus metric names, and curl examples: Usage guide (full) §10.

The binary wire protocol on the primary port is byte-untouched whether the HTTP gateway runs or not.