fix(macos): enable x86_64 cross-compilation

- Upgrade rust_cast 0.18.1 → 0.21 (switches from openssl to rustls,
  removing the openssl-sys dependency that blocked x86_64 cross-compile)
- Split mupdf sys-lib features to Linux-only (on macOS there are no GTK
  symbol conflicts, and system libs don't support x86_64 cross-compile
  from Apple Silicon)
- Update release-macos workflow to build both aarch64 and x86_64 DMGs
  using a matrix strategy

crates/qbz-cast/Cargo.toml

@@ -6,7 +6,7 @@ description = "Casting support for QBZ - Chromecast, AirPlay, DLNA"
 
 [dependencies]
 # Chromecast
-rust_cast = "0.18.1"
+rust_cast = "0.21"
 
 # mDNS discovery
 mdns-sd = "0.11"