rustclipsync-core 0.4.0

rustclipsync-core 0.4.0

This is a breaking Rust API and wire-protocol upgrade. Use RustClipSync Server
3.0.0, Desktop 0.5.0 or Omaclipsync 0.4.0 together. Older clients require server
2.0.0. Mobile applications and Fcitx input methods are released separately.

• Add protocol-3 device direction reports, separate pause state, WebSocket ACKs,
  authenticated HTTP fallback and fenced session/version negotiation.
• Receive only authorized per-device deliveries; reconnect backfill is the last
  eligible message within the server's configured window. Do not hardcode it.
• Add background receive APIs without reading clipboard snapshots; writes remain
  explicit opt-ins, and sensitive-content policy remains enforced.
• Keep ordinary receives working after marked-sensitive synchronization is disabled.
• Add opt-in durable conflict cancellation/skip recovery and detection of legacy
  outboxes before receive. Preserve local clipboard and reject stale retries;
  hosts retain responsibility for choosing recovery policy and fencing events.
• Preserve existing device identities and RCS1 encrypted state. Older state
  defaults the recovery marker to false. Rust API additions and required transport
  methods are a pre-1.0 minor API break; endpoint prefix remains /api/.

Validation: 68 Rust tests, rustfmt, strict Clippy, rustdoc, and actual core/server
HTTPS interoperability with real WSS and forced HTTP fallback. Consumer native
acceptance is documented in the respective repositories. This release is source
on GitHub; the crate is not published to crates.io.