relay: TCP listener, framing encoders and a stdlib Python client for the sidecar stream

[TheDrOnos]

relay: TCP listener, framing encoders and a stdlib Python client for the sidecar stream

Summary

Adds the transport pieces an app needs to expose its backend stream over TCP, with no
policy of their own:

• de_shell/js/main/framing.ts — encodeMessage / encodeBinary, the inverse of
  stdoutDemux.ts. Byte-identical framing to ipc.emit and anyplotlib's encode_frame.
• de_shell/js/main/relay.ts — createRelay({host, port, onConnection, onLine, onClose, …}).
  Binds exactly the given address (an empty host is refused; 0.0.0.0 only when asked),
  hands each client line to the app as a string (the first one included, so the app owns
  admission), guards every connection until the app calls admit() (hello timeout, a
  small pre-admission line cap, a larger one after), writes framed units per connection
  with the payload as its own chunk, exposes writableLength for the app's backpressure
  decisions, closes gracefully so a written refusal reaches the client, and sets keepalive
  and no-delay on accepted sockets. Imports only node:net, so it runs under node --test.
  A createServer factory option leaves room for TLS without an API change.
• de_shell/remote_client.py — a synchronous, stdlib-only client: connect, send,
  recv (("message", dict) / ("binary", header, bytes) / ("stream", str)), close.
  recv waits on a selector so a send on another thread is never cut short. Malformed
  units raise ValueError with the offending bytes on .line, after being consumed.
• Barrel re-exports; tests/test_boundary.py gains the new module; one towncrier fragment.

Nothing in backendProcess.ts or stdoutDemux.ts changes. Existing apps are unaffected
until they call the new exports.

Consumers

• Ground Crew's remote-control endpoint (directelectron/de_ground_crew#321): pairing,
  tokens, admission, the action filter and snapshots all stay in GC; the endpoint is a thin
  use of createRelay, and GC's laptop-side connector uses remote_client.
• spyde #159 (backend on a cluster, GUI on the laptop): the same encoders and client are
  the codec for its serve process and connect mode; both stay yours.

Deliberately not in this PR

Connect mode for Electron, a Python serve relay, a subscriber API on backendProcess
(Ground Crew tees in its own handlers), any auth or policy, TLS itself (only the factory
hook), an onServerError hook for post-listen server errors (currently swallowed so
EMFILE cannot crash main — worth adding when an app wants the log line).

Two calls left to you: whether host should additionally require an IP literal (a
hostname is DNS-resolved to whichever address comes first); and, with TLS, relay.close()
would wait for sockets still in handshake, which the relay never sees — a doc note or a
close timeout when the hook is first used.

Tests

• framing.test.ts (8): encoders decoded through the production demuxer, at several chunk
  sizes, with marker look-alikes and \n inside payloads, non-ASCII headers, non-finite
  numbers refused.
• relay.test.ts (32): real loopback sockets on port 0 — exact bind and OS-error rejection,
  empty host refused, lines across chunk splits and \r\n, hello timeout and both caps on
  an injected clock, two clients delivered (no count of its own), writableLength under a
  stalled reader, graceful close delivering a queued refusal before EOF, destroy after the
  grace, relay.close() races, keepalive / no-delay / createServer injection observed
  through a spy factory.
• tests/test_remote_client.py (20): the decoder fed real ipc.emit / encode_frame bytes
  over a socketpair, whole and at 1- and 7-byte steps; recv timeout keeps partial bytes;
  EOF → ConnectionError; malformed prefix/header/JSON → ValueError with .line; a poll
  on one thread beside a blocking send on another; and a cross-runtime round trip against a
  real createRelay under Node (skips by name when node is absent or cannot strip types).

Evidence

• Windows (Node 24.16.0, Python 3.14): npm run typecheck clean · npm run test:unit
  133/133 · pytest 117 passed, 0 skipped · wheel carries the three new source files.
• Linux (Node 24.19.0, Python 3.13): npm run typecheck clean · npm run test:unit
  133/133 · pytest 117 passed, 0 skipped. (npm 11 withheld electron's postinstall under
  allow-scripts; neither suite needs the binary.)

Release

New public API on both halves; a 0.4.0 release would let apps pin it
(de-shell>=0.4,<0.5). The fragment is upcoming_changes/+relay.new_feature.rst; it will
be renamed to <this PR's number>.new_feature.rst once the number exists.