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/caps.rs

@@ -40,6 +40,14 @@ pub const VIDEO_CAP_PROBE_SEQ: u8 = 0x10;
 /// button/axis events; toward a host that doesn't set the bit it keeps the legacy events.
 pub const HOST_CAP_GAMEPAD_STATE: u8 = 0x01;
 
+/// [`Welcome::host_caps`] bit: the host has a shared-clipboard service (a working OS backend)
+/// **and** its operator policy does not hard-disable it, so the client may offer the clipboard
+/// toggle. Absent (an older host, or `PUNKTFUNK_CLIPBOARD` off) ⇒ the client greys the toggle
+/// out. Purely additive: nothing clipboard-related happens until a [`ClipControl`]`{ enabled:
+/// true }` crosses (see `design/clipboard-and-file-transfer.md` §3.1). Packs into the existing
+/// trailing `host_caps` byte — no wire-layout change.
+pub const HOST_CAP_CLIPBOARD: u8 = 0x02;
+
 /// [`Hello::video_codecs`] bit: the client can decode H.264 / AVC. The GPU-less **software**
 /// encode path (openh264) emits H.264, so a client that wants to stream from a software host MUST
 /// advertise this.