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-core/src/quic/mod.rs

@@ -52,6 +52,10 @@ pub mod endpoint;
 /// Async framed-message IO over a quinn stream (`u16 LE length || payload`).
 pub mod io;
 
+/// Per-transfer clipboard fetch bi-streams (`PKFs` magic + kind byte, then request/response). The
+/// transport half of the shared clipboard; wire codecs are in [`control`], state lives per side.
+pub mod clipstream;
+
 /// SPAKE2 over Ed25519 for the pairing ceremony. The two roles use the asymmetric flow so
 /// the identities are ordered; each side binds **both** certificate fingerprints as the
 /// SPAKE2 identities, so the derived key only matches when client and host agree on the PIN