Launchers: the start file now upgrades packages that are out of date

Suite 2.246.7. Tool versions are unchanged.

What changed

Out-of-date Python packages are upgraded at launch. The start file used to check only that the packages could be imported, so a package that was installed long ago and still imported was never looked at. It now compares each installed package with the minimum version in requirements.txt and, when one is missing or older, runs pip install --upgrade -r requirements.txt before the app starts. Pillow is the package where this matters most: it decodes the floor-plan images inside .esx files.
A failed upgrade does not block the launch when everything is installed. If every package is present but one is older than its minimum and the upgrade cannot run (for example, no network), the start file prints a warning and starts with the versions installed. A missing package still stops the launch with the install command to run by hand.
The macOS start file now also covers Pillow, which its old check left out.

The exit code now says which case applies: 0 all current, 1 something missing, 2 everything installed but something older than its minimum. The installers still treat any non-zero code as "update".

A new test runs the dependency step of each start file against a stand-in Python and checks the four outcomes: current (pip not run), missing or older with a successful upgrade, older with a failed upgrade (launch continues), and missing with a failed install (launch stops). It fails when the upgrade flag or the continue-on-older branch is removed.
The full suite passes.