feat(clients): reachability-probed presence + shareable Decky host management

Add a bounded, trust-agnostic, mDNS-INDEPENDENT QUIC reachability probe and
surface it everywhere saved-host presence is shown, so a host reached over a
routed network (Tailscale/VPN/multicast-filtering LAN) no longer reads Offline
just because it isn't advertising — the display-side companion to the 0.8.4
dial-first connect fix.

Core:
- punktfunk-core: NativeClient::probe (bounded handshake; a real host answers even
  on trust mismatch, a wrong/closed/TCP-only port fails) + punktfunk_probe C ABI
  (ABI_VERSION 3->4, header regenerated).
- pf-client-core: trust::probe_reachable_many (parallel per-host sweep).

Presence pips now read `advertising OR probed-reachable`, refreshed by a ~10-12s
background sweep off the UI thread:
- Linux (relm4): ui_hosts probed map + HostsMsg::Probed sweep.
- Windows (windows-reactor): pf-probe worker -> HostsProps.probed.
- Apple (SwiftUI): HostStore.refreshReachability, driven by HomeView + GamepadHomeView .task.
- Android (Compose): nativeProbe JNI seam + periodic LaunchedEffect (LNP-gated),
  online dot added to the touch HostCard.
- Decky already probes via --list-hosts --probe.

Decky client: make the flatpak client's known-hosts store the single source of
truth via new headless CLI modes (--list-hosts / --add-host / --set-host /
--forget-host / --reset / --reachable). The plugin can now add a host by address,
edit/forget hosts, reset all state (keeping the client identity), and shows
probe-backed online pips — state is shared with the desktop client, not duplicated.

Also lands in-progress Android 17 LNP groundwork (targetSdk 37 +
ACCESS_LOCAL_NETWORK runtime flow, permission dialogs) that was already present in
the working tree.

Verified: cargo check + clippy clean (punktfunk-core, pf-client-core, linux,
android native); android assembleDebug BUILD SUCCESSFUL; decky typecheck + rollup
build clean; probe true/false-positive behaviour exercised against a live host.
Windows and Apple were not compiled locally (no MSVC/Xcode on this Linux box).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/Stores/HostStore.swift

@@ -81,6 +81,11 @@ final class HostStore: ObservableObject {
         didSet { persist() }
     }
 
+    /// Saved hosts proven reachable by the periodic QUIC probe (by id) — the mDNS-independent
+    /// counterpart to discovery presence, OR'd into the "online" pip so a routed/VPN host that
+    /// never advertises still reads Online. Not persisted (it's live reachability, not config).
+    @Published var probedOnline: Set<StoredHost.ID> = []
+
     init() {
         if let data = UserDefaults.standard.data(forKey: Self.key),
            let decoded = try? JSONDecoder().decode([StoredHost].self, from: data) {
@@ -110,6 +115,23 @@ final class HostStore: ObservableObject {
         hosts[i].lastConnected = Date()
     }
 
+    /// One reachability sweep, driving `probedOnline`: probe every saved host NOT currently
+    /// advertising on `discovery` (mDNS already answers for those) off the main actor via a bounded,
+    /// trust-agnostic QUIC handshake, then publish the reachable set. This is the mDNS-independent
+    /// half of presence — a host reached over a routed network (Tailscale/VPN) never advertises but
+    /// answers here. Call in a loop from a home view's `.task` (cancelled on disappear).
+    func refreshReachability(discovery: HostDiscovery) async {
+        let targets = hosts.filter { !discovery.advertises($0) }
+        var online: Set<StoredHost.ID> = []
+        for host in targets {
+            let reachable = await Task.detached(priority: .utility) {
+                PunktfunkConnection.probe(host: host.address, port: host.port)
+            }.value
+            if reachable { online.insert(host.id) }
+        }
+        probedOnline = online
+    }
+
     func pin(_ hostID: UUID, fingerprint: Data) {
         guard let i = hosts.firstIndex(where: { $0.id == hostID }) else { return }
         hosts[i].pinnedSHA256 = fingerprint