packaging: build against packaged ltk 0.2 and enable arm64 cross-builds
Release 0.4.0. Resolve the ltk crate from the libltk-dev package's cargo registry (/usr/share/cargo/registry/ltk-0.2.0) instead of an in-tree ../ltk checkout, and bump the build dependency to libltk-dev (>= 0.2.0, << 0.3). Drop the unused forge_ipc workspace dependency: no member crate activated it and it never appeared in Cargo.lock — leftover from moving the greeter out of forge.
Make the package cross-buildable for arm64 from amd64. The build is driven by a hand-rolled Makefile rather than dh-cargo, so the cross toolchain is wired manually:
  * Makefile gains CARGO_TARGET: when set it adds --target and switches the install source from target/release to target/<triple>/release, so install picks up the cross-built binaries. Empty keeps native builds unchanged.
  * debian/rules detects host != build via architecture.mk, maps the Debian arch to its Rust triple, and exports the cross linker (CARGO_TARGET_<TRIPLE>_LINKER), CC_<triple>, and a host-arch PKG_CONFIG with PKG_CONFIG_ALLOW_CROSS so the -sys crates (pam, xkbcommon, wayland) resolve against the host architecture.
  * debian/control build-deps are now cross-satisfiable: cargo/rustc marked :native, link libraries narrowed to the ones actually linked (libpam0g-dev, libxkbcommon-dev, libwayland-dev). Removed libclang-dev (no bindgen in the tree) and replaced libpam-dev with libpam0g-dev. EGL/GLES are loaded via khronos-egl/glow with the dynamic feature (dlopen at runtime), so they are not build dependencies.
  * arm64 / arm64-deps helpers updated to use the cross linker and pkg-config and to pull crossbuild-essential-arm64.
Build with: dpkg-buildpackage --host-arch arm64 -b
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