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/Session/StreamCommands.swift

@@ -21,6 +21,12 @@ import SwiftUI
 /// `.focusedSceneValue` so the Scene-level commands can drive it.
 struct SessionFocus {
     var isStreaming: Bool
+    /// The connected host advertises `HOST_CAP_CLIPBOARD` (gates the Share Clipboard item —
+    /// macOS-only UI, but the fact is platform-neutral).
+    var clipboardAvailable: Bool
+    /// Clipboard sync is live (host-acked) — drives the item's Stop/Share title.
+    var clipboardOn: Bool
+    var toggleClipboard: () -> Void
     var disconnect: () -> Void
 }
 
@@ -58,6 +64,15 @@ struct StreamCommands: Commands {
             }
             .keyboardShortcut("q", modifiers: [.control, .option, .shift])
             .disabled(session?.isStreaming != true)
+            #if os(macOS)
+            // Mid-session clipboard flip (design/clipboard-and-file-transfer.md §5.3). Greyed
+            // when the host doesn't advertise the cap (older host / operator policy off).
+            Button(session?.clipboardOn == true ? "Stop Sharing Clipboard" : "Share Clipboard") {
+                session?.toggleClipboard()
+            }
+            .keyboardShortcut("c", modifiers: [.control, .option, .shift])
+            .disabled(session?.isStreaming != true || session?.clipboardAvailable != true)
+            #endif
             Divider()
             Button("Disconnect") { session?.disconnect() }
                 .keyboardShortcut("d", modifiers: [.control, .option, .shift])