Network Type Transition Optimization

Nucleic-Session: 40F8C53A-F001-423A-8607-EAE181DBE14B
Co-authored-by: Nucleic <[email protected]>

NucleicRemote/NucleicRemote/Net/NetworkPathMonitor.swift

@@ -0,0 +1,51 @@
+import Foundation
+import Network
+
+/// Watches the device's network path (Wi‑Fi ⇄ cellular ⇄ offline) so the app can switch transports
+/// the instant the link changes, instead of waiting for a dead LAN socket to time out. This is the
+/// missing proactive signal behind the "iOS sticks on LAN after leaving Wi‑Fi" stall: the phone's
+/// pinned `lanHost:lanPort` is only reachable over Wi‑Fi/wired, so on cellular the LAN candidate is a
+/// guaranteed stall. `HostConnection` skips LAN when `canUseLAN` is false and re‑plans on `onChange`.
+@MainActor
+final class NetworkPathMonitor: ObservableObject {
+    /// True when a LAN‑capable path (Wi‑Fi or wired Ethernet) is available — the only paths on which
+    /// the pinned `lanHost:lanPort` could resolve. Cellular‑only ⇒ false, so the LAN candidate is
+    /// dropped and we dial tailnet/relay straight away. Defaults optimistic so a not‑yet‑started
+    /// monitor never spuriously suppresses LAN.
+    @Published private(set) var canUseLAN = true
+    /// True when any network path is available at all (used to decide whether an immediate reconnect
+    /// is even worth attempting).
+    @Published private(set) var isSatisfied = true
+
+    /// Fired on the main actor whenever a *relevant* aspect of the path changed — LAN availability or
+    /// overall reachability. The cue for connections to re‑plan their transport immediately rather
+    /// than waiting out a reconnect backoff.
+    var onChange: (() -> Void)?
+
+    private let monitor = NWPathMonitor()
+    private let queue = DispatchQueue(label: "nucleic.remote.path")
+    private var started = false
+
+    func start() {
+        guard !started else { return }
+        started = true
+        monitor.pathUpdateHandler = { [weak self] path in
+            Task { @MainActor in self?.apply(path) }
+        }
+        monitor.start(queue: queue)
+    }
+
+    /// Re‑read the current path synchronously — used on foreground, where an update that landed while
+    /// the app was suspended may not have been redelivered yet.
+    func refresh() { apply(monitor.currentPath) }
+
+    private func apply(_ path: NWPath) {
+        let lan = path.status == .satisfied
+            && (path.usesInterfaceType(.wifi) || path.usesInterfaceType(.wiredEthernet))
+        let satisfied = path.status == .satisfied
+        let changed = lan != canUseLAN || satisfied != isSatisfied
+        canUseLAN = lan
+        isSatisfied = satisfied
+        if changed { onChange?() }
+    }
+}