merge(core): reconcile the W7/W8 client refactor with origin's shared-clipboard feature

origin/main landed the shared clipboard (design/clipboard-and-file-transfer.md) while
this branch split quic/msgs.rs -> quic/{caps,control,...} and client.rs ->
client/{mod,control,worker,pump,planes,...} (W7) and deleted the two monoliths. The
feature had modified both deleted files, so its delta is re-applied onto the split
instead of resurrecting the monoliths:

  - HOST_CAP_CLIPBOARD                         -> quic/caps.rs
  - MSG_CLIP_* / CLIP_* consts, the six Clip*
    structs, and their encode/decode impls     -> quic/control.rs (beside the clock codecs)
  - CtrlRequest::{ClipControl,ClipOffer} +
    Negotiated.host_caps                        -> client/control.rs
  - WorkerArgs.{clip_event_tx,clip_cmd_rx}      -> client/worker.rs
  - CLIP_EVENT_QUEUE                            -> client/planes.rs
  - NativeClient clip fields, the 7 clip_* /
    host_caps / next_clip methods, connect()
    channel wiring                              -> client/mod.rs
  - the control-task encode/decode arms and
    the clipboard-task spawn                     -> client/pump.rs

Cargo.lock reconciled (adds pf-clipboard), punktfunk-host/Cargo.toml unions the W6
pf-* subsystem deps with pf-clipboard, and include/punktfunk_core.h is the cbindgen
union (clipboard + rumble C-ABI). punktfunk-core builds --all-features and its 174
lib tests pass, including quic::tests::clip_loopback.

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

crates/punktfunk-host/src/native/handshake.rs

@@ -365,8 +365,16 @@ pub(super) async fn negotiate(
         // assuming HEVC.
         codec: codec_bit,
         // This host applies sequence-gated gamepad-state snapshots (InputKind::GamepadState),
-        // so capable clients send those instead of the loss-fragile per-transition events.
-        host_caps: punktfunk_core::quic::HOST_CAP_GAMEPAD_STATE,
+        // so capable clients send those instead of the loss-fragile per-transition events. The
+        // clipboard bit is advertised only when the operator policy enables it (design
+        // clipboard-and-file-transfer.md §3.1) AND this platform has a backend — see
+        // `pf_clipboard::cap_advertised` for the deliberate compositor-lacks-data-control case.
+        host_caps: punktfunk_core::quic::HOST_CAP_GAMEPAD_STATE
+            | if pf_clipboard::cap_advertised() {
+                punktfunk_core::quic::HOST_CAP_CLIPBOARD
+            } else {
+                0
+            },
     };
     io::write_msg(send, &welcome.encode()).await?;