Add Debian package target

[thoughtbox]

Summary

• configure CPack to generate Debian packages on Linux
• add a deb target for packaging an already-finished build
• detect the package architecture and shared-library dependencies automatically
• stage existing build artifacts without triggering CPack's recursive preinstall rebuild

Usage

cmake -S . -B build -DCUTTER_ENABLE_PACKAGING=ON
cmake --build build
cmake --build build --target deb

The generated package is written to the build directory using Debian naming conventions.

Verification

Tested end-to-end on Debian ARM64:

CPack: Create package using DEB
CPack: Install projects
CPack: Create package
CPackDeb: - Generating dependency list
CPack: - package: /home/torh/git/cutter/build/cutter_2.5.0-dev-d7f11b2_arm64.deb generated.
[100%] Built target deb

Also verified with dpkg-deb that the package:

• reports package name cutter, version 2.5.0-dev-d7f11b2, and architecture arm64
• contains /usr/bin/cutter and re.rizin.cutter.desktop
• declares the automatically detected Qt 6 and system-library dependencies

AI Disclosure

OpenCode using GPT-5.6 assisted with implementing the CMake/CPack changes, diagnosing the initial CPack staging issue, and running the verification commands. The resulting code and generated Debian package were reviewed and tested locally.