feat(apple/clipboard): macOS client half of the shared clipboard (Phase 1 §5)

The NSPasteboard bridge completing Phase 1 (design/clipboard-and-file-transfer.md
§5) — with the host backends on this branch, copy/paste now crosses the wire in
both directions on macOS. Lazy in both directions:

- PunktfunkConnection grows the clipboard plane: its own clipboardLock (close()
  joins it like the other pullers), hostCaps/hostSupportsClipboard from the
  Welcome, the typed ClipEvent vocabulary, and the six ABI wrappers
  (clipControl/clipOffer/clipFetch/clipServe/clipCancel/nextClipboard — borrowed
  event payloads copied out before the next poll).
- ClipboardSync (PunktfunkKit, macOS-only): one drain thread bridging
  NSPasteboard.general ↔ the QUIC clipboard plane. Local copies announce format
  lists via a 500 ms changeCount poll (+ immediate on app activation); bytes
  leave only on a host FetchRequest, answered from the live pasteboard and
  seq-guarded against staleness. Host copies install one NSPasteboardItem whose
  data provider fires only when a Mac app actually pastes, then blocks its
  provider thread (never main) on a 10 s-bounded fetch. Concealed/Transient
  pasteboards (password managers) are never announced; our own writes are
  changeCount-suppressed (§3.4). Text/RTF/HTML/PNG; files ride Phase 2.
- UI: per-host "Share clipboard with this host" toggle (StoredHost.clipboardSync,
  optional for saved-JSON forward-compat — wire-format tests extended), a
  mid-session Share/Stop Sharing Clipboard item in the Stream menu (⌃⌥⇧C,
  greyed without HOST_CAP_CLIPBOARD), SessionModel owning the lifecycle
  (start on streaming after the trust gate, drain joined off-main on teardown).

swift build + swift test green (macOS). Requires the ABI v8 xcframework
(scripts/build-xcframework.sh).

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

clients/apple/Sources/PunktfunkClient/Home/AddHostSheet.swift

@@ -20,6 +20,12 @@ struct AddHostSheet: View {
     @State private var address: String
     @State private var port: Int
     @State private var mac: String
+    #if os(macOS)
+    /// Share the clipboard with this host (macOS sessions only; design
+    /// clipboard-and-file-transfer.md §5.3). Off by default; honored only when the host
+    /// advertises the capability at connect.
+    @State private var clipboardSync: Bool
+    #endif
     #if os(tvOS)
     private enum EditField: String, Identifiable {
         case name, address, port, mac
@@ -41,6 +47,9 @@ struct AddHostSheet: View {
         _port = State(initialValue: Int(existing?.port ?? 9777))
         let stored = existing?.macAddresses ?? []
         _mac = State(initialValue: (stored.isEmpty ? suggestedMacs : stored).joined(separator: ", "))
+        #if os(macOS)
+        _clipboardSync = State(initialValue: existing?.clipboardSync ?? false)
+        #endif
     }
 
     var body: some View {
@@ -96,6 +105,9 @@ struct AddHostSheet: View {
                     #if os(iOS)
                     .textInputAutocapitalization(.never)
                     #endif
+                #if os(macOS)
+                Toggle("Share clipboard with this host", isOn: $clipboardSync)
+                #endif
             }
             #if !os(tvOS)
             .formStyle(.grouped)
@@ -147,6 +159,11 @@ struct AddHostSheet: View {
         host.address = address.trimmingCharacters(in: .whitespaces)
         host.port = UInt16(clamping: port)
         host.macAddresses = Self.parseMacs(mac)
+        #if os(macOS)
+        // nil when off: the key stays absent from the saved JSON (forward-compat, and "never
+        // opted in" and "opted out" read the same — off).
+        host.clipboardSync = clipboardSync ? true : nil
+        #endif
         onSave(host)
         dismiss()
     }