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/Tests/PunktfunkKitTests/SharedFoundationTests.swift

@@ -18,7 +18,7 @@ final class SharedFoundationTests: XCTestCase {
             name: "Tower", address: "192.168.1.173", port: 9777,
             pinnedSHA256: Data([0xDE, 0xAD, 0xBE, 0xEF]),
             lastConnected: Date(timeIntervalSince1970: 1_700_000_000),
-            mgmtPort: 47990, macAddresses: ["aa:bb:cc:dd:ee:ff"])
+            mgmtPort: 47990, macAddresses: ["aa:bb:cc:dd:ee:ff"], clipboardSync: true)
 
         let data = try JSONEncoder().encode(host)
         let decoded = try JSONDecoder().decode(StoredHost.self, from: data)
@@ -39,6 +39,7 @@ final class SharedFoundationTests: XCTestCase {
         XCTAssertNil(decoded.macAddresses)
         XCTAssertNil(decoded.pinnedSHA256)
         XCTAssertNil(decoded.lastConnected)
+        XCTAssertNil(decoded.clipboardSync)
         // Resolvers fall back cleanly.
         XCTAssertEqual(decoded.effectiveMgmtPort, punktfunkDefaultMgmtPort)
         XCTAssertEqual(decoded.wakeMacs, [])