feat: tracker-as-first-layer-node inference entry point (US-014)

- Tracker: add GET /v1/tracker-nodes/<model> returning nodes registered
  with tracker_mode=true whose shard_start matches the model's first layer
- Node: StubNodeServer and TorchNodeServer accept tracker_mode/tracker_url;
  when tracker_mode=True (or auto-detected via shard_start==0 for Torch),
  /v1/chat/completions is served alongside /forward
- TorchNodeServer: full pipeline implementation — encode_prompt → route
  selection via tracker → binary forward through remaining hops → decode
- Gateway: _handle_chat_completions checks _get_tracker_nodes() first and
  proxies round-robin to tracker-nodes; falls back to existing direct
  pipeline when none found (preserves all US-005 backward compat)
- CLI: --tracker-mode and --tracker-url flags added to meshnet-node start
- Test: two stub tracker-nodes + two mid-shard nodes for gpt2; 10 requests;
  round-robin 5/5 split verified; all OpenAI-format responses validated
- All 78 tests pass

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

.scratch/distributed-inference-network/prd.json

@@ -346,13 +346,14 @@
         "Commit only this story's changes"
       ],
       "priority": 14,
-      "passes": false,
+      "passes": true,
       "notes": "Source issue: .scratch/distributed-inference-network/issues/14-tracker-as-node.md",
       "dependsOn": [
         "US-012",
         "US-013"
       ],
-      "status": "open"
+      "status": "done",
+      "completionNotes": "Implemented: (1) Tracker GET /v1/tracker-nodes/<model> endpoint returning nodes registered with tracker_mode=true at shard_start==0. (2) StubNodeServer and TorchNodeServer now accept tracker_mode/tracker_url params; when tracker_mode=True, /v1/chat/completions is served alongside /forward. (3) TorchNodeServer auto-detects tracker mode when shard_start==0. (4) Gateway _handle_chat_completions checks for tracker-nodes first via _get_tracker_nodes(), proxies round-robin if found, falls back to existing direct pipeline if none (backward compat). (5) CLI --tracker-mode and --tracker-url flags added. (6) Integration test: 2 tracker-nodes + 2 mid-shard nodes for gpt2; 10 requests; round-robin verified (5/5 split); all responses valid OpenAI format. All 78 tests pass."
     },
     {
       "id": "US-015",