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>

crates/punktfunk-core/src/abi.rs

@@ -1436,6 +1436,39 @@ pub unsafe extern "C" fn punktfunk_generate_identity(
     })
 }
 
+/// Reachability probe: attempt the QUIC handshake to `host:port` and report whether the host
+/// answered — trust-agnostic and mDNS-INDEPENDENT. A host reached over a routed network
+/// (Tailscale/VPN/another subnet) answers here even though it never advertises on mDNS, so the
+/// clients' saved-host "online" pips can reflect real reachability instead of LAN presence (the
+/// display-side companion to the dial-first connect fix). Returns [`PunktfunkStatus::Ok`] when
+/// reachable, [`PunktfunkStatus::Timeout`] when not (or on any connect error). Blocks up to
+/// `timeout_ms`; call off the UI thread.
+///
+/// # Safety
+/// `host` must be a NUL-terminated UTF-8 string.
+#[cfg(feature = "quic")]
+#[no_mangle]
+pub unsafe extern "C" fn punktfunk_probe(
+    host: *const std::os::raw::c_char,
+    port: u16,
+    timeout_ms: u32,
+) -> PunktfunkStatus {
+    guard(|| {
+        let Ok(Some(host)) = (unsafe { opt_cstr(host) }) else {
+            return PunktfunkStatus::NullPointer;
+        };
+        if crate::client::NativeClient::probe(
+            host,
+            port,
+            std::time::Duration::from_millis(timeout_ms as u64),
+        ) {
+            PunktfunkStatus::Ok
+        } else {
+            PunktfunkStatus::Timeout
+        }
+    })
+}
+
 /// Run the PIN pairing ceremony against a host (see the protocol docs in punktfunk-core):
 /// the host displays a short PIN; the user types it into the client app, which passes it
 /// here. On success the host has stored this client's identity, the now-verified host