Rename Castle -> Wild PC across the repo

Repo-side rename only (Phases 1-3 of the migration plan); the live box
(~/.castle, systemd units, /data/castle, domains) is a separate cutover.

- Slug `castle` -> `wildpc`: CLI command, module names (wildpc_core/cli/api),
  dist names, entry point `wildpc = wildpc_cli.main:main`.
- Identifiers: CastleConfig/NATSClient/DirError/MDNS -> Wildpc*.
- Env/constants: CASTLE_* -> WILDPC_*; ~/.castle -> ~/.wildpc, castle.yaml ->
  wildpc.yaml, /data/castle -> /data/wildpc.
- Systemd UNIT_PREFIX castle- -> wildpc-; own programs castle-api/gateway/etc.
- Display prose "Castle" -> "Wild PC" in docs, agent-guide files, README, frontend.
- Package dirs and bootstrap yaml renamed via git mv; lockfiles regenerated;
  redundant nested uv.lock files dropped (workspace root lock is authoritative).

Tests: core 273, cli 47, wildpc-api 120 all pass. Frontend type-checks + builds.
Fixed a stale test fixture (secret_env_path kind arg) broken pre-rename.

wildpc-api/src/wildpc_api/mesh.py

@@ -0,0 +1,78 @@
+"""Mesh state manager — aggregates remote node registries."""
+
+from __future__ import annotations
+
+import logging
+import time
+from dataclasses import dataclass, field
+
+from wildpc_core.registry import NodeRegistry
+
+logger = logging.getLogger(__name__)
+
+# Remote registries older than this are considered stale.
+STALE_TTL_SECONDS = 300  # 5 minutes
+
+
+@dataclass
+class RemoteNode:
+    """A remote node's registry and metadata."""
+
+    registry: NodeRegistry
+    last_seen: float = field(default_factory=time.time)
+    online: bool = True
+
+    @property
+    def is_stale(self) -> bool:
+        return (time.time() - self.last_seen) > STALE_TTL_SECONDS
+
+
+class MeshStateManager:
+    """Singleton holding remote node state discovered via MQTT.
+
+    Thread-safe for reads from the FastAPI request handlers.
+    Mutations happen only from the MQTT callback task.
+    """
+
+    def __init__(self) -> None:
+        self._nodes: dict[str, RemoteNode] = {}
+
+    def update_node(self, hostname: str, registry: NodeRegistry) -> None:
+        """Add or update a remote node's registry."""
+        self._nodes[hostname] = RemoteNode(registry=registry)
+        logger.info(
+            "Mesh: updated node %s (%d deployed)", hostname, len(registry.deployed)
+        )
+
+    def set_offline(self, hostname: str) -> None:
+        """Mark a node as offline (LWT received)."""
+        if hostname in self._nodes:
+            self._nodes[hostname].online = False
+            logger.info("Mesh: node %s went offline", hostname)
+
+    def remove_node(self, hostname: str) -> None:
+        """Remove a node entirely."""
+        if self._nodes.pop(hostname, None):
+            logger.info("Mesh: removed node %s", hostname)
+
+    def get_node(self, hostname: str) -> RemoteNode | None:
+        """Get a specific remote node."""
+        return self._nodes.get(hostname)
+
+    def all_nodes(self, *, include_stale: bool = False) -> dict[str, RemoteNode]:
+        """Return all remote nodes, optionally filtering out stale ones."""
+        if include_stale:
+            return dict(self._nodes)
+        return {h: n for h, n in self._nodes.items() if not n.is_stale}
+
+    def prune_stale(self) -> list[str]:
+        """Remove nodes that have gone stale. Returns list of pruned hostnames."""
+        pruned = [h for h, n in self._nodes.items() if n.is_stale]
+        for h in pruned:
+            del self._nodes[h]
+            logger.info("Mesh: pruned stale node %s", h)
+        return pruned
+
+
+# Module-level singleton — imported by MQTT client and API routes.
+mesh_state = MeshStateManager()