feat(mesh): fleet role + shared config/presence KV (Phase 2)

- add static 'role' (authority|follower) to NodeConfig/CastleConfig, wired from
  castle.yaml through the registry to the mesh wire; civil pinned authority
- castle-presence: TTL KV bucket each node renews (churn signal); delete-on-stop
- castle-config: authority-gated get/put + change-watch SSE (follower reconcile
  hook hangs off it)
- bound the NATS drain on stop() so shutdown can't hang
- tests: role config/registry round-trip, wire round-trip, role write-gating

Single-node verified; follower reconcile pending the second node.

castle-api/tests/test_nats_client.py

@@ -0,0 +1,30 @@
+"""CastleNATSClient role gating — hermetic (no live NATS server needed).
+
+The write-gate is checked before touching the KV bucket, so a follower is denied
+without any connection.
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+import pytest
+from castle_core.registry import NodeConfig, NodeRegistry
+
+from castle_api.nats_client import CastleNATSClient
+
+
+def _client(role: str) -> CastleNATSClient:
+    reg = NodeRegistry(node=NodeConfig(hostname="n", role=role), deployed={})
+    return CastleNATSClient("n", reg, servers="nats://localhost:4222")
+
+
+def test_role_property() -> None:
+    assert _client("authority").role == "authority"
+    assert _client("follower").role == "follower"
+
+
+def test_follower_cannot_write_shared_config() -> None:
+    client = _client("follower")
+    with pytest.raises(PermissionError):
+        asyncio.run(client.put_shared_config("fleet/key", "value"))