feat(us-016): smart shard gap detection for auto-join and --model-id

Tracker /v1/network/assign now accepts an optional `hf_repo` query param
to restrict assignment to a specific model, and returns `gap_found: bool`
so callers know whether they received a real gap vs a redundancy slot.

Node startup with --model-id (no explicit shard args) now queries the
tracker first for an uncovered gap for that model before defaulting to
full coverage (0..n-1). This means a second node with --model-id will
serve only the missing layers, not the whole model again.

Auto-join fallback (no --model-id) now prints why it fell through
instead of silently switching to stub-model.

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

tests/test_node_startup.py

@@ -507,6 +507,73 @@ def test_second_node_downloads_same_shard_from_peer_without_huggingface(
         tracker.stop()
 
 
+def test_network_assign_gap_found_field():
+    """network/assign sets gap_found=True when a real gap exists, False when fully covered."""
+    import json as _json
+    import urllib.request as _ur
+
+    tracker = TrackerServer()
+    port = tracker.start()
+    try:
+        # Register a node covering only layers 0-11 of a 24-layer model.
+        data = _json.dumps({
+            "endpoint": "http://127.0.0.1:9200",
+            "model": "Qwen2.5-0.5B-Instruct",
+            "hf_repo": "Qwen/Qwen2.5-0.5B-Instruct",
+            "num_layers": 24,
+            "shard_start": 0,
+            "shard_end": 11,
+            "hardware_profile": {},
+            "score": 1.0,
+        }).encode()
+        req = _ur.Request(
+            f"http://127.0.0.1:{port}/v1/nodes/register",
+            data=data,
+            headers={"Content-Type": "application/json"},
+            method="POST",
+        )
+        with _ur.urlopen(req) as r:
+            r.read()
+
+        # A new node should be told there is a gap (layers 12-23).
+        resp = _get_json(
+            f"http://127.0.0.1:{port}/v1/network/assign?device=cpu&vram_mb=0"
+            "&hf_repo=Qwen/Qwen2.5-0.5B-Instruct"
+        )
+        assert resp["gap_found"] is True
+        assert resp["shard_start"] == 12, f"expected gap at 12, got {resp['shard_start']}"
+        assert resp["shard_end"] == 23
+
+        # Register the second node covering the gap.
+        data2 = _json.dumps({
+            "endpoint": "http://127.0.0.1:9201",
+            "model": "Qwen2.5-0.5B-Instruct",
+            "hf_repo": "Qwen/Qwen2.5-0.5B-Instruct",
+            "num_layers": 24,
+            "shard_start": 12,
+            "shard_end": 23,
+            "hardware_profile": {},
+            "score": 1.0,
+        }).encode()
+        req2 = _ur.Request(
+            f"http://127.0.0.1:{port}/v1/nodes/register",
+            data=data2,
+            headers={"Content-Type": "application/json"},
+            method="POST",
+        )
+        with _ur.urlopen(req2) as r:
+            r.read()
+
+        # Now fully covered — gap_found should be False.
+        resp2 = _get_json(
+            f"http://127.0.0.1:{port}/v1/network/assign?device=cpu&vram_mb=0"
+            "&hf_repo=Qwen/Qwen2.5-0.5B-Instruct"
+        )
+        assert resp2["gap_found"] is False
+    finally:
+        tracker.stop()
+
+
 def test_startup_cpu_fallback(tmp_path, monkeypatch):
     """Node starts with CPU warning when no GPU is detected."""
     import meshnet_node.startup as startup_mod