Add opt-in client/server mode to the rdx executable

[paracycle]

What

Adds an opt-in resident, per-workspace server to the rdx executable. The server keeps an indexed + resolved Rubydex::Graph in memory so repeated rdx query invocations skip the index/resolve cost. Without --server (or on platforms lacking fork/UNIX sockets) rdx query runs inline exactly as before.

Stacked on top of #868. Review/merge that first.

This is the first step of a larger effort to turn rdx into a client/server tool (similar in spirit to the RuboCop and Spring servers), so that the expensive indexing + resolution work is paid once and reused across commands. It is intentionally scoped to the foundations: the runtime/IPC plumbing and a working server for the read-only query path. It's pure Ruby on top of the existing gem API — no Rust/C changes.

CLI surface

• rdx query <CYPHER> [--server] [--format ...] — opt into the resident server for a query (otherwise runs inline).
• rdx server <start|stop|restart|status> [--no-detach] — manage the resident server for the current workspace.
• Env: DISABLE_RDX_SERVER (force inline) and RDX_SERVER_DIR (override the runtime directory).

How

• lib/rubydex/cli.rb — extracts all command/option parsing and subcommand dispatch into a Rubydex::CLI class, keeping exe/rdx a thin shim. Heavyweight requires (rubydex, rubydex/server) stay deferred into the handlers so paths that don't need the native extension stay cheap to start.
• lib/rubydex/server/cache.rb — per-workspace runtime dir keyed by an app-id hashed from the workspace path, Ruby/rubydex version and a native-extension fingerprint (a gem upgrade forces a fresh server). Manages pid/token/socket/version files with 0700/0600 perms.
• lib/rubydex/server/request.rb — length-prefixed JSON framing + an IO.select handshake read timeout.
• lib/rubydex/server/core.rb — UNIX socket accept loop with a version-line gate, constant-time token auth, mutex-guarded dispatch (query/status/stop) and per-request mtime-manifest staleness (reindex changed / delete_document removed / resolve).
• lib/rubydex/server/client.rb — start-if-absent (fork + Process.daemon, flock single-start, spawn-and-poll readiness), version handshake, cold/warm recovery, graceful stop with SIGTERM fallback.
• lib/rubydex/server/commands.rb — implements the rdx server start|stop|restart|status actions.

The socket is bound/connected via a relative name from its directory so long runtime paths stay under the sockaddr_un ~104-byte limit.

What's coming next

This PR deliberately keeps the scope small. Follow-up PRs will build on this foundation:

• Warm rdx console sessions served from the resident graph. An IRB session needs the client's real terminal, so this requires forking a per-session worker, passing the client's STDIN/STDOUT/STDERR file descriptors to it, and forwarding signals — landing a REPL that starts instantly against the already-built graph.
• Always-fresh graph via a background file watcher. Instead of the per-request staleness check this PR uses, a watcher thread will apply incremental updates as files change, plus idle-timeout self-shutdown.

Testing

• Unit tests for Cache (app-id stability, perms, version compare, stale cleanup) and the request framing.
• Integration tests that spawn a real server subprocess: warm query, staleness on add/delete, inline-equivalence, start/status/stop, restart. POSIX-only assertions skip on Windows (where server mode is unsupported).
• bundle exec rake ruby_test — all Ruby tests pass; RuboCop clean.