ci(deb): publish punktfunk-host .deb to the Gitea apt registry

Wires up the half-built Debian packaging: build-deb.sh existed but nothing
invoked or published it. Adds a `deb` workflow that builds the release host in
the Ubuntu 26.04 rust-ci image, packages it (dpkg-shlibdeps-resolved Depends,
NVIDIA driver filtered out), and uploads to Gitea's public Debian registry on
every main push (rolling 0.0.1~ciN.<sha>) and v* tag (clean X.Y.Z). Ubuntu hosts
then track it with `apt update && apt upgrade`.

Also: box-setup docs (packaging/debian/README.md), a pointer from the packaging
README, ignore dist/, and drop backticks from the package Description (the
unquoted control heredoc ran them as a command substitution).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/debian/build-deb.sh

@@ -123,7 +123,7 @@ Description: Low-latency desktop/game streaming host (Moonlight + punktfunk/1)
  .
  NVENC + GPU EGL come from the NVIDIA driver (libnvidia-encode / libEGL_nvidia),
  installed out of band. After install: add yourself to the 'input' group for virtual
- gamepads, then `systemctl --user enable --now punktfunk-host`.
+ gamepads, then enable the systemd user service punktfunk-host.
 EOF
 
 cat > "$STAGE/DEBIAN/postinst" <<'EOF'