Task: Implement the AppImage Reader feature

TASK-DESCRIPTION

• Create sources/appimage/appimage_reader.h.
• Create sources/appimage/appimage_reader.cpp.
• Create sources/appimage/squashfs_reader.h.
• Create sources/appimage/squashfs_reader.cpp.
• Create the portable test tests/30-appimage-reader.sh.
• Create the tool-gated test tests/40-squashfs-payload-reader.sh.
• Implement the requirements in prompts/features/03-appimage-reader.md.
• Run make test from the repository root.

TASK-OUTPUT

• Report the created files and the result of make test.

TASK-CONTEXT

• Feature requirements: prompts/features/03-appimage-reader.md.
• Format facts and the payload offset algorithm: documents/07-appimage-format.md.
• The SquashFS reader may use the host zlib, liblzma, and libzstd libraries.
• The test fixture is a real ELF from the host with the AppImage magic written at offset 8 and a SquashFS image appended at the computed payload offset.

TASK-FILES

• sources/appimage/appimage_reader.h — new
• sources/appimage/appimage_reader.cpp — new
• sources/appimage/squashfs_reader.h — new
• sources/appimage/squashfs_reader.cpp — new
• tests/30-appimage-reader.sh — new
• tests/40-squashfs-payload-reader.sh — new