Prefactor: shared OTLP http/json server machinery

[philcunliffe]

Parent

Implementation spec: #798 (LLP 0245).

What to build

Make the change easy before making the easy change: extract the OTLP http/json server mechanics that today live inside the @hypaware/otel plugin (per-signal routing, json-only content-type enforcement with 415 otherwise, gzip/deflate content-encoding, OTLP partialSuccess envelopes, loopback bind-and-resolve) into shared machinery that more than one plugin can host a listener on. @hypaware/otel consumes the shared machinery with zero behavior change; payload interpretation stays plugin-owned.

Acceptance criteria

• @hypaware/otel runs on the extracted machinery with no config or wire-behavior change
• Existing otel smokes green (listener-writes-rows, loopback capture)
• Self-telemetry loop guard behavior unchanged
• npm test green; no new public config surface

Blocked by

None - can start immediately.