feat(us-024): enhanced availability map with per-node health details

Coverage endpoint now skips node purge so dead nodes remain visible.
Each band in /v1/coverage/<model> includes a `nodes` array with per-node
health: alive status, last_seen_seconds_ago, heartbeat_success_rate,
inference_success_rate, queue_depth, uptime_seconds, and endpoint.
Updated tests to assert new band structure.

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

tests/test_tracker_routing.py

@@ -243,10 +243,24 @@ def test_tracker_coverage_endpoint_reports_uncovered_ranges():
         )
 
         coverage_resp = _get_json(f"http://127.0.0.1:{tracker_port}/v1/coverage/tiny-model")
-        assert coverage_resp["coverage"] == [
-            {"start_layer": 0, "end_layer": 2, "node_count": 1},
-            {"start_layer": 3, "end_layer": 5, "node_count": 0},
-        ]
+        bands = coverage_resp["coverage"]
+        assert len(bands) == 2
+        # Covered band: layers 0-2, one node
+        assert bands[0]["start_layer"] == 0
+        assert bands[0]["end_layer"] == 2
+        assert bands[0]["node_count"] == 1
+        assert len(bands[0]["nodes"]) == 1
+        node_info = bands[0]["nodes"][0]
+        assert node_info["endpoint"] == "http://127.0.0.1:9001"
+        assert node_info["alive"] is True
+        assert "last_seen_seconds_ago" in node_info
+        assert "heartbeat_success_rate" in node_info
+        assert "inference_success_rate" in node_info
+        # Gap band: layers 3-5, no nodes
+        assert bands[1]["start_layer"] == 3
+        assert bands[1]["end_layer"] == 5
+        assert bands[1]["node_count"] == 0
+        assert bands[1]["nodes"] == []
 
         try:
             _get_json(f"http://127.0.0.1:{tracker_port}/v1/route?model=tiny-model")
@@ -283,11 +297,12 @@ def test_tracker_coverage_endpoint_accepts_registered_hf_repo_or_short_name():
         )
 
         assert by_repo["model"] == "Qwen/Qwen2.5-0.5B-Instruct"
-        assert by_repo["coverage"] == [
-            {"start_layer": 0, "end_layer": 19, "node_count": 1},
-            {"start_layer": 20, "end_layer": 21, "node_count": 2},
-            {"start_layer": 22, "end_layer": 23, "node_count": 1},
-        ]
+        bands = by_repo["coverage"]
+        assert len(bands) == 3
+        assert bands[0]["start_layer"] == 0 and bands[0]["end_layer"] == 19 and bands[0]["node_count"] == 1
+        assert bands[1]["start_layer"] == 20 and bands[1]["end_layer"] == 21 and bands[1]["node_count"] == 2
+        assert bands[2]["start_layer"] == 22 and bands[2]["end_layer"] == 23 and bands[2]["node_count"] == 1
+        assert all("nodes" in b for b in bands)
         assert by_short_name["coverage"] == by_repo["coverage"]
     finally:
         tracker.stop()