feat(clipboard): wire protocol + client-core task for shared clipboard (Phase 0)

The portable shared-clipboard plane in punktfunk-core, all behind the `quic`
feature (design/clipboard-and-file-transfer.md §3):

- Control messages 0x40–0x44 (ClipControl / ClipOffer / ClipFetch...) and the
  HOST_CAP_CLIPBOARD capability bit, negotiated in the Welcome caps.
- Per-transfer QUIC bi-streams ("PKFs" magic) for lazy fetch of offered content,
  with ClipFetchHdr status/size framing (quic::clipstream).
- The §3.5 portable wire-MIME vocabulary (text/plain;utf-8, text/html, text/rtf,
  image/png) shared by both ends.
- Client-side clipboard task (client.rs) + C ABI surface bumped to v8 (abi.rs,
  regenerated include/punktfunk_core.h).
- Loopback transport tests (quic::tests).

No OS clipboard integration yet — that is the host backends (Phase 1/3) and the
macOS client (Phase 1).

Ported from feat/shared-clipboard (af3a7d8c, pre-W6 base) onto current main;
three deliberate deviations from the original commit:
- ABI v6 → v8: main took v6 (reanchor gate) and v7 (typed connect rejection)
  in the meantime; the clipboard C surface re-lands as v8.
- CLIP_CANCELLED_CODE 0x60 → 0x70: main's pairing-rejection close codes claimed
  the 0x60–0x67 block; the vocabularies stay disjoint on purpose.
- Negotiated.host_caps coexists with main's 6-tuple host_caps plumbing: main
  needs the worker-local copy for gamepad snapshots, the clipboard path needs it
  across ready_tx to build the NativeClient handle (punktfunk_connection_host_caps).

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

crates/punktfunk-core/src/lib.rs

@@ -30,6 +30,11 @@ mod abr;
 pub mod audio;
 #[cfg(feature = "quic")]
 pub mod client;
+/// Client-side shared-clipboard transport: the per-session task that runs the fetch-stream accept
+/// loop, drives outbound fetches, and serves inbound ones — surfaced to the embedder as poll
+/// events. Wire codecs live in [`quic`]; the OS pasteboard integration lives in the native client.
+#[cfg(feature = "quic")]
+pub mod clipboard;
 pub mod config;
 pub mod crypto;
 pub mod error;
@@ -73,7 +78,11 @@ pub use stats::Stats;
 /// application close) and the `PunktfunkStatus` −20 block itself. Additive — the close codes are
 /// new application-close vocabulary an old peer simply never sends/reads, so [`WIRE_VERSION`] is
 /// unchanged.
-pub const ABI_VERSION: u32 = 7;
+/// v8: added the shared-clipboard client surface — `punktfunk_connection_host_caps` and
+/// `punktfunk_connection_clipboard_{control,offer,fetch,serve,cancel}` +
+/// `punktfunk_connection_next_clipboard`. Additive; the wire grows only backward-compatible control
+/// messages (0x40-0x44) and a new `Welcome::host_caps` bit, so [`WIRE_VERSION`] is unchanged.
+pub const ABI_VERSION: u32 = 8;
 
 /// The punktfunk/1 **wire** version — what `Hello`/`Welcome` carry and hosts equality-check.
 /// Deliberately its own constant: [`ABI_VERSION`] tracks the embeddable **C surface**