setup: recreate an incomplete .venv instead of activating it

A failed venv creation (e.g. the Debian/Ubuntu python3-venv package
missing) left the .venv directory behind; the next run saw it, skipped
creation and failed to source/activate a venv without activate/pip.
setup.sh now validates bin/activate, bin/python and bin/pip, removes an
incomplete directory and creates it again; a creation failure removes the
partial directory and names the venv package.  setup.bat does the same
with Scripts\activate.bat/python.exe/pip.exe and !errorlevel!.

READMEs (prerequisites + scripts table), CHANGELOG (folded into the
unreleased 3.38.1) and AGENTS updated.

Tests: tests/test_setup_scripts.py (5: the shell/bat source pins and
bash -n).  1088 Python + 1075 frontend.

setup.sh

@@ -14,10 +14,24 @@ command -v git >/dev/null 2>&1 || { echo "Error: Git is required but not found.
 PY_VER=$(python3 -c 'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}")')
 echo "Found Python $PY_VER"
 
-# Create venv
+# Create the venv.  A failed creation (missing venv/ensurepip) leaves a partial
+# .venv behind: validate it and remove it so the next run starts over (v3.38.1).
+if [ -d "$VENV_DIR" ] && { [ ! -f "$VENV_DIR/bin/activate" ] || [ ! -x "$VENV_DIR/bin/python" ] || [ ! -x "$VENV_DIR/bin/pip" ]; }; then
+    echo "Removing an incomplete virtual environment (a previous setup did not finish)..."
+    rm -rf "$VENV_DIR"
+fi
 if [ ! -d "$VENV_DIR" ]; then
     echo "Creating virtual environment..."
-    python3 -m venv "$VENV_DIR"
+    if ! python3 -m venv "$VENV_DIR"; then
+        rm -rf "$VENV_DIR"
+        echo "Error: could not create the virtual environment."
+        echo "  The Python venv/ensurepip component is missing or broken."
+        echo "    Debian/Ubuntu: sudo apt install python3-venv"
+        echo "    Fedora:        sudo dnf install python3-libs"
+        echo "    Arch:          venv and ensurepip are part of python"
+        echo "  Install it and run ./setup.sh again."
+        exit 1
+    fi
 fi
 source "$VENV_DIR/bin/activate"
 PIP="$VENV_DIR/bin/pip"