Add Tailscale (Tailnet) as a sync transport between Mac and iPhone

Settings ▸ Remote gains a "Connect via" picker — LAN (default), Tailscale
(tailnet), or Relay (disabled, coming soon). On Tailnet, both devices run an
embedded tsnet node via TailscaleKit (tailscale/libtailscale) and sync frames
flow over the user's tailnet, so the phone can connect from anywhere the
tailnet reaches; Noise E2EE runs above the transport unchanged.

- NucleicTailnet (new target, macOS + iOS): TailnetNode wraps TailscaleKit's
  node lifecycle (auth-key login, generation-fenced start/stop since up() is
  un-cancellable) and drops to the framework's public C API for the data path
  — tailscale_dial/listen/accept hand back full-duplex socketpair fds, wrapped
  by FDFrameChannel (DispatchIO) into the shared FrameChannel seam. The Swift
  wrapper's one-way connection actors can't carry a bidirectional stream.
- Host: TailnetListener adopts SyncListener; startSyncServer is single-flight
  and honors toggle-off/picker changes at the commit point; pairing QRs carry
  transport + tailnet IP/port hints (PairingPayload additive optional fields,
  forward/backward compatible over CBOR).
- iPhone: pair/reconnect dial over whichever transport the pairing recorded;
  Settings gains a Tailscale auth-key field (Keychain, committed on editing
  end); connectivity chip shows "Connected · Tailnet".
- TailscaleKit has no SwiftPM distribution: scripts/build-tailscalekit.sh
  builds a pinned libtailscale commit into an untracked local xcframework;
  Package.swift links it only when present (everything builds without it, the
  picker then reports Tailscale support as not built in), and the script
  clears SwiftPM's content-keyed manifest cache so the toggle is picked up.
- iOS floor 17.0 → 18.1 (TailscaleKit requires the iOS 18 Swift runtime);
  package-app.sh embeds the framework in the .app like Sparkle.

703-test suite: no new failures (the 7 fake-claude/fake-grok staging issues
reproduce identically on an untouched checkout — pre-existing, tracked
separately). New coverage: FDFrameChannel over socketpairs, pairing-payload
version-skew both directions, transport-setting resolution.

Co-Authored-By: Claude Fable 5 <[email protected]>

NucleicRemote/NucleicRemote/Models/IdentityStore.swift

@@ -3,8 +3,10 @@ import Security
 import NucleicProtocol
 
 /// What the phone pins about its Mac at pairing (SYNC §4.2): the host's static key (for IK
-/// reconnect), a display name, and an optional LAN hint. The pairing secret is *not* stored —
-/// it's one-time. Non-secret, so UserDefaults is fine; the device private key goes to Keychain.
+/// reconnect), a display name, and the transport + connection hint from the QR — LAN
+/// host:port or the Mac's tailnet IP. The pairing secret is *not* stored — it's one-time.
+/// Non-secret, so UserDefaults is fine; the device private key goes to Keychain. The new
+/// optional fields decode as nil from a pre-transport record (= LAN).
 struct PairedHost: Codable, Equatable {
     var deviceID: String
     var hostName: String
@@ -12,6 +14,12 @@ struct PairedHost: Codable, Equatable {
     var fingerprint: String
     var lanHost: String?
     var lanPort: UInt16?
+    /// `SyncTransportHint` raw value; nil = LAN (records saved before transports existed).
+    var transport: String?
+    var tailnetHost: String?
+    var tailnetPort: UInt16?
+
+    var transportHint: SyncTransportHint { transport.flatMap(SyncTransportHint.init(rawValue:)) ?? .lan }
 }
 
 /// Loads/persists this device's long-term `DeviceIdentity` (Keychain) and the pinned host