feat(us-016): outbound relay client for NAT/internet pipeline hops

Nodes behind NAT (5G, WSL2, home routers) can now participate in
distributed pipeline inference over the internet via the relay server.

- torch_server: add module-level _relay_hop() that opens a WebSocket
  to relay.../rpc/{peer_id}, sends the binary activation with
  body_base64 encoding, and returns (status, headers, body)
- torch_server: _get_remaining_route returns list[dict] (was list[tuple])
  preserving relay_addr from injected X-Meshnet-Route header and
  from /v1/route slow-path node info
- torch_server: _run_downstream_pipeline dispatches via _relay_hop
  when hop has relay_addr; falls back to direct HTTP on relay error
- tracker server: downstream_hops dicts include relay_addr when node
  has one registered, so head node knows how to reach each peer
- relay_bridge: binary bodies (bfloat16 activations) use body_base64;
  response preserves all X-Meshnet-* headers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/node/meshnet_node/relay_bridge.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import base64
 import json
 import logging
 import threading
@@ -114,20 +115,35 @@ class RelayHttpBridge:
         request_id = str(payload.get("request_id") or "")
         method = str(payload.get("method") or "POST").upper()
         path = str(payload.get("path") or "/")
-        body_text = payload.get("body") or ""
         headers = payload.get("headers") if isinstance(payload.get("headers"), dict) else {}
 
+        # body_base64 carries binary data (e.g. bfloat16 activation tensors) safely.
+        # Fallback to text "body" for backward-compat with non-binary requests.
+        body_b64 = payload.get("body_base64")
+        if body_b64:
+            data = base64.b64decode(body_b64)
+        else:
+            body_text = payload.get("body") or ""
+            data = body_text.encode() if isinstance(body_text, str) else bytes(body_text)
+
         url = f"{self.local_base_url}{path}"
-        data = body_text.encode() if isinstance(body_text, str) else bytes(body_text)
         req = urllib.request.Request(url, data=data, headers=headers, method=method)
         try:
             with urllib.request.urlopen(req, timeout=300.0) as resp:
-                return {
+                resp_bytes = resp.read()
+                resp_headers = dict(resp.headers)
+                # Forward all X-Meshnet-* headers so the caller can reconstruct the activation.
+                is_binary = "octet-stream" in resp.headers.get("Content-Type", "")
+                result: dict = {
                     "request_id": request_id,
                     "status": resp.status,
-                    "headers": {"Content-Type": resp.headers.get("Content-Type", "application/json")},
-                    "body": resp.read().decode(errors="replace"),
+                    "headers": resp_headers,
                 }
+                if is_binary:
+                    result["body_base64"] = base64.b64encode(resp_bytes).decode()
+                else:
+                    result["body"] = resp_bytes.decode(errors="replace")
+                return result
         except urllib.error.HTTPError as exc:
             return {
                 "request_id": request_id,