feat(meshcore): add data model, connection config, MeshcoreClient skeleton

Implements utils/meshcore.py with all dataclasses (MeshcoreMessage,
MeshcoreNode, MeshcoreContact, MeshcoreTelemetry, MeshcoreTraceroute),
connection configs (SerialConfig, TCPConfig, BLEConfig), ConnectionState
enum, serial port discovery, and the MeshcoreClient singleton skeleton.
Adds tests/test_meshcore_client.py covering all dataclasses, availability
check, and state enum (8/8 tests passing).

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

tests/test_meshcore_client.py

@@ -0,0 +1,97 @@
+"""Tests for MeshcoreClient dataclasses and state machine."""
+
+from datetime import datetime, timezone
+from unittest.mock import patch
+
+
+class TestAvailability:
+    def test_returns_bool(self):
+        from utils.meshcore import is_meshcore_available
+
+        assert isinstance(is_meshcore_available(), bool)
+
+    def test_false_when_not_installed(self):
+        with patch.dict("sys.modules", {"meshcore": None}):
+            import importlib
+
+            import utils.meshcore as m
+
+            importlib.reload(m)
+            assert m.is_meshcore_available() is False
+
+
+class TestMeshcoreMessage:
+    def _make(self, **kw):
+        from utils.meshcore import MeshcoreMessage
+
+        defaults = {
+            "id": "abc123",
+            "sender_id": "NODE001",
+            "recipient_id": "BROADCAST",
+            "text": "hello mesh",
+            "timestamp": datetime(2026, 5, 10, 12, 0, 0, tzinfo=timezone.utc),
+            "hop_count": 2,
+            "snr": -8.5,
+            "is_direct": False,
+        }
+        defaults.update(kw)
+        return MeshcoreMessage(**defaults)
+
+    def test_to_dict_keys(self):
+        d = self._make().to_dict()
+        for key in ("id", "sender_id", "recipient_id", "text", "timestamp", "hop_count", "snr", "is_direct", "pending"):
+            assert key in d, f"missing key: {key}"
+
+    def test_pending_defaults_false(self):
+        assert self._make().to_dict()["pending"] is False
+
+    def test_none_snr_allowed(self):
+        d = self._make(snr=None).to_dict()
+        assert d["snr"] is None
+
+
+class TestMeshcoreNode:
+    def test_to_dict_includes_is_repeater(self):
+        from utils.meshcore import MeshcoreNode
+
+        node = MeshcoreNode(
+            node_id="RPT1",
+            name="Roof-Repeater",
+            is_repeater=True,
+            lat=51.5,
+            lon=-0.1,
+            battery_pct=87,
+            last_seen=datetime.now(timezone.utc),
+            snr=-5.0,
+            hops_away=1,
+        )
+        d = node.to_dict()
+        assert d["is_repeater"] is True
+        assert d["node_id"] == "RPT1"
+
+
+class TestMeshcoreTelemetry:
+    def test_to_dict_timestamp_is_iso(self):
+        from utils.meshcore import MeshcoreTelemetry
+
+        t = MeshcoreTelemetry(
+            node_id="N1",
+            timestamp=datetime(2026, 5, 10, tzinfo=timezone.utc),
+            battery_pct=72,
+            voltage=3.7,
+            temperature=22.1,
+            humidity=55.0,
+            uptime_secs=3600,
+        )
+        d = t.to_dict()
+        assert "2026-05-10" in d["timestamp"]
+
+
+class TestConnectionState:
+    def test_state_enum_values(self):
+        from utils.meshcore import ConnectionState
+
+        assert ConnectionState.DISCONNECTED
+        assert ConnectionState.CONNECTING
+        assert ConnectionState.CONNECTED
+        assert ConnectionState.ERROR