The .deb recipe already lives in Cargo.toml's [package.metadata.deb], but the
build *environment* (bookworm distrobox, glibc floor, the mandatory separate
CARGO_TARGET_DIR) was only captured in handoff notes. Add a packaging/debian
README so the deb path is as self-documenting as the Arch + AppImage paths.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
