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.

core/tests/test_fleet_role.py

@@ -0,0 +1,48 @@
+"""Fleet role (authority/follower) — config loading + registry round-trip."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import yaml
+from castle_core.config import load_config
+from castle_core.registry import (
+    NodeConfig,
+    NodeRegistry,
+    load_registry,
+    save_registry,
+)
+
+
+def _write_min_config(root: Path, role: str | None) -> None:
+    data: dict = {"gateway": {"port": 18000}}
+    if role is not None:
+        data["role"] = role
+    (root / "castle.yaml").write_text(yaml.safe_dump(data))
+
+
+def test_role_defaults_to_follower(tmp_path: Path) -> None:
+    _write_min_config(tmp_path, None)
+    assert load_config(tmp_path).role == "follower"
+
+
+def test_role_loaded_from_yaml(tmp_path: Path) -> None:
+    _write_min_config(tmp_path, "authority")
+    assert load_config(tmp_path).role == "authority"
+
+
+def test_registry_role_round_trip(tmp_path: Path) -> None:
+    reg = NodeRegistry(
+        node=NodeConfig(hostname="civil", role="authority"), deployed={}
+    )
+    path = tmp_path / "registry.yaml"
+    save_registry(reg, path)
+    assert load_registry(path).node.role == "authority"
+
+
+def test_registry_role_defaults_follower(tmp_path: Path) -> None:
+    """A node with no explicit role round-trips as follower."""
+    reg = NodeRegistry(node=NodeConfig(hostname="n"), deployed={})
+    path = tmp_path / "registry.yaml"
+    save_registry(reg, path)
+    assert load_registry(path).node.role == "follower"