Switch Dataiku DSS extension to Unix sockets for proxy communication

[hanneshapke]

Scope

Tracking issue only — the Dataiku DSS extension code lives in a separate repo. Implementation will happen there; this issue exists so the work is visible alongside the rest of the kiji-proxy roadmap.

Motivation

The Dataiku DSS extension currently talks to the proxy over a TCP loopback port. Switching to a Unix domain socket would:

• Remove the need to pick / reserve a TCP port on the DSS host (avoids conflicts with other services).
• Eliminate exposure on the loopback interface — sockets are gated by filesystem permissions, so only the DSS user (or an explicit group) can connect.
• Slightly lower latency / overhead vs. TCP for same-host IPC.

Proposal

• Update the DSS extension to connect to the proxy via a Unix socket path (configurable, with a sensible default like /var/run/kiji-proxy.sock or $XDG_RUNTIME_DIR/kiji-proxy.sock).
• Ensure the proxy side supports listening on a Unix socket (verify / add as needed in this repo).
• Document socket permissions (owner, group, mode) so DSS can read/write but unrelated users can't.
• Provide a fallback / migration note for existing DSS deployments still on TCP.

Acceptance criteria

• DSS extension connects via Unix socket by default.
• Socket path is configurable via the extension settings / env var.
• Proxy-side Unix socket listener verified to work end-to-end with DSS.
• Docs updated (DSS extension repo + a pointer from this repo's README if relevant).

Notes

• Windows DSS hosts don't have first-class Unix socket support; keep TCP as a fallback there.
• Implementation PRs will land in the DSS extension repo, not here.