fix(macos): rebuild incomplete venv instead of failing on re-run (#3106)

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

start-macos.sh

@@ -130,11 +130,12 @@ fi
 # 3. Python environment + dependencies (kept inside the repo, in venv/).
 #    Named `venv` to match the manual steps and build-macos-app.sh, so the
 #    clickable .app reuses this same environment.
-if [ ! -d venv ]; then
+VENV_PY="./venv/bin/python3"
+if [ ! -x "$VENV_PY" ] || ! "$VENV_PY" -m pip --version >/dev/null 2>&1; then
+    [ -d venv ] && { echo "▶ Existing venv is incomplete (no working pip) — rebuilding…"; rm -rf venv; }
     echo "▶ Creating Python environment…"
     "$PY" -m venv venv
 fi
-VENV_PY="./venv/bin/python3"
 REQ_HASH="$(md5 -q requirements.txt 2>/dev/null || md5sum requirements.txt | cut -d' ' -f1)"
 REQ_HASH_FILE="venv/.requirements_hash"
 if [ ! -f "$REQ_HASH_FILE" ] || [ "$REQ_HASH" != "$(cat "$REQ_HASH_FILE" 2>/dev/null)" ]; then