The installed state of a project is judged by the proofs of installation,
which differ per subpackage: make/project.conf in the dev tree or /opt
proves the -devel package or a buildable checkout, a VERSION file in the
project directory or /usr/share/doc/packages the -run package.
Add is_installed(), which answers the proof question for a given subpackage
by searching the per-subpackage locations.
Signed-off-by: Jan Lindemann <jan@janware.com>
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.84.2