start-macos.sh guarded venv creation with `[ ! -d venv ]`, which trusts any
existing venv/ directory even when a prior run was interrupted before pip was
bootstrapped into it. Re-runs then failed with "No module named pip" and never
self-healed, contradicting the script's "safe to re-run" promise.
Validate that the venv has a working pip before reusing it, and rebuild it
otherwise.
Fixes#3105
