scratch worlds from a seed, so a run does not depend on a hand-made save

[Flatts3000]

The problem

Verification currently depends on a hand-made world that has to exist, be named correctly, and be in
the right state. --world "New World" assumes somebody made it. A world that drifts (blocks placed
during a previous run, time of day, a wandering mob) makes two runs differ for reasons unrelated to
the change under test.

The proposal

Create a scratch world on demand from a fixed seed, and throw it away afterwards:

gamebridge launch --instance "$INSTANCE" --port "$PORT" --new-world --seed 12345 --scratch

--scratch deletes the save on exit, so a run leaves nothing behind and the next one starts
identical.

Where this belongs

Probably the launcher rather than the mod, since world creation is a launch concern and the game is
not running yet. Vanilla has --quickPlaySingleplayer for an existing world but no "create one";
that may mean writing a level.dat before launch, which is a real piece of work and needs care.

Notes

• The alternative is a template world copied into place before launch, which is cruder and probably
  works today with no code at all. Worth trying that first and seeing whether it is enough.
• Interacts with the freeze issue: a fixed seed plus frozen weather and time is most of what
  reproducible screenshots need.