Launcher preview foundation for Ubuntu

[fallintoplace]

Summary

This is the first real implementation branch for the repository.

It adds the launcher, provider contract docs, Debian packaging skeleton, smoke tests, CI, and contributor scaffolding needed to review the first working Ubuntu preview.

Why

I wanted main to stay small and readable while the first substantial review happens in one place.

This branch also hardens the runtime trust model:

• only launcher-verified runtimes are reusable or stoppable
• unknown healthy loopback services are discarded instead of reused
• fresh runtime starts reopen the app instead of focusing a stale window

Validation

• make test
• make build-deb

Notes

• main is intentionally a minimal landing branch
• the current runtime path is launcher-first and browser-shell based