Add .deb packaging for Debian/Ubuntu

[kramerc]

Follow-up to #16 (RPM + MSI packaging). Debian/Ubuntu users currently have no first-class install path — they'd need to use the dev artifact pipeline or build from source. This issue tracks adding .deb support.

Tooling

cargo-deb is the Rust ecosystem standard, analogous to the cargo-generate-rpm we already use on the RPM side. Reads metadata from Cargo.toml, ships a single self-contained .deb from the built binary.

[package.metadata.deb]
maintainer = "Kramer Campbell"
extended-description = """
Discord Rich Presence IPC proxy that multiplexes RPC messages across
multiple Discord instances."""
section = "net"
priority = "optional"
assets = [
    ["target/release/presence-switch", "usr/bin/", "755"],
    ["packaging/linux/systemd/presence-switch.service", "usr/lib/systemd/user/", "644"],
    ["LICENSE", "usr/share/doc/presence-switch/copyright", "644"],
]

(Some duplication with the RPM asset list; a small refactor could share asset definitions if it grows.)

Per-user installation question

Unlike RPM/MSI, .deb installation via apt / dpkg is system-wide and requires sudo. The file payload would still install a user-level systemd unit at /usr/lib/systemd/user/presence-switch.service (each user enables via systemctl --user enable --now), so the runtime model stays per-user. Only the install step is privileged. Worth calling out in the README.

Sketch of work

• Add [package.metadata.deb] block to Cargo.toml
• Add a deb job to .github/workflows/package.yml, parallel to the existing rpm job:
  • cargo install cargo-deb --locked
  • cargo build --release --locked
  • cargo deb --no-build (with release tag override for dev builds)
• Update scripts/package.sh to add a deb subcommand
• Update the release job to attach the .deb to the GitHub Release
• README: add apt install ./presence-switch_*.deb example