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.

tests/test_setup_scripts.py

@@ -0,0 +1,63 @@
+"""The setup scripts recover from an incomplete .venv (v3.38.1).
+
+A failed venv creation (the Debian/Ubuntu python3-venv package missing) used to
+leave the .venv directory behind; the next run saw it, skipped creation and
+failed to activate a venv without activate/pip.
+"""
+
+import shutil
+import subprocess
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+class SetupShTests(unittest.TestCase):
+    def setUp(self):
+        self.src = (ROOT / 'setup.sh').read_text()
+
+    def test_an_incomplete_venv_is_removed_before_creation(self):
+        self.assertIn('Removing an incomplete virtual environment', self.src)
+        self.assertIn('rm -rf "$VENV_DIR"', self.src)
+        self.assertIn('bin/activate', self.src)
+        self.assertIn('bin/python', self.src)
+        self.assertIn('bin/pip', self.src)
+
+    def test_a_failed_creation_removes_the_partial_dir_and_guides(self):
+        self.assertIn('if ! python3 -m venv "$VENV_DIR"; then', self.src)
+        self.assertIn('python3-venv', self.src)
+        fails = self.src[self.src.index('if ! python3 -m venv'):]
+        self.assertLess(fails.index('rm -rf "$VENV_DIR"'),
+                        fails.index('could not create the virtual environment'))
+
+    def test_the_script_is_valid_bash(self):
+        bash = shutil.which('bash')
+        if not bash:
+            self.skipTest('bash not available')
+        res = subprocess.run([bash, '-n', str(ROOT / 'setup.sh')],
+                             capture_output=True, text=True)
+        self.assertEqual(res.returncode, 0, res.stderr)
+
+
+class SetupBatTests(unittest.TestCase):
+    def setUp(self):
+        self.src = (ROOT / 'setup.bat').read_text()
+
+    def test_an_incomplete_venv_is_removed_before_creation(self):
+        self.assertIn('Removing an incomplete virtual environment', self.src)
+        self.assertIn('rmdir /s /q "%VENV_DIR%"', self.src)
+        for marker in ('Scripts\\activate.bat', 'Scripts\\python.exe',
+                       'Scripts\\pip.exe'):
+            self.assertIn(marker, self.src)
+
+    def test_a_failed_creation_removes_the_partial_dir_and_guides(self):
+        self.assertIn('if !errorlevel! neq 0', self.src)
+        self.assertIn('python -m venv "%VENV_DIR%"', self.src)
+        fails = self.src[self.src.index('if !errorlevel! neq 0'):]
+        self.assertLess(fails.index('rmdir /s /q "%VENV_DIR%"'),
+                        fails.index('could not create the virtual environment'))
+
+
+if __name__ == '__main__':
+    unittest.main()