fix: fail Linux browser preflight before download

[abrichr]

What changed

• openadapt doctor now checks Chromium's Linux system libraries before any browser download.
• Four case-sensitive X11 sonames are corrected before the launcher calls the current released Flow runtime.
• Missing browser libraries and missing core packages return a nonzero exit status with a repair command from the active Python interpreter.
• The public quickstart keeps the browser download lazy. Its clean-wheel lifecycle starts with an empty browser cache, installs Linux host libraries only, runs doctor, and proves that the first browser action downloads Chromium.
• The README and installation docs now state the Linux host-library and network requirements. They no longer promise that the tutorial needs no extra system packages.

This change keeps the current public Flow and Agent dependency ranges. It does not wait for a new cross-package release train.

Checks

• Linux clean-wheel lifecycle: passed on exact commit d831c31 with current public dependencies
• pytest: 293 passed, 1 skipped
• ruff check openadapt tests scripts/quickstart_lifecycle.py
• ruff format --check openadapt tests scripts/quickstart_lifecycle.py
• python scripts/validate_platform_manifest.py --offline
• python scripts/verify_release_lock.py
• python scripts/check_source_boundary.py
• python scripts/render_readme_maturity.py --check