Set PYTHONUTF8=1 in generated Windows env files (#3)

Adds PYTHONUTF8=1 to env.ps1 (Windows only) and env.bat so that
Python defaults to UTF-8 encoding in managed-python sessions.

Co-authored-by: Oliver C <178891725+oliver-cieliszak-natterbox@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: oliver-cieliszak-natterbox

setup.py

@@ -202,8 +202,10 @@ def _write_env_ps1(prefix: Path, uv_env: str, python_env: str, distro_version: s
         "# Env vars (always set -- these are the reliable contract)",
         f'$env:{uv_env} = "{uv_exe}"',
         f'$env:{python_env} = "{venv_py}"',
-        "",
     ]
+    if _IS_WINDOWS:
+        lines.append('$env:PYTHONUTF8 = "1"')
+    lines.append("")
 
     if add_to_path:
         lines += [f"# {n}" for n in notes]
@@ -240,6 +242,7 @@ def _write_env_bat(prefix: Path, uv_env: str, python_env: str, distro_version: s
         ":: Env vars (always set -- these are the reliable contract)",
         f'SET {uv_env}={uv_exe}',
         f'SET {python_env}={venv_py}',
+        'SET PYTHONUTF8=1',
         "",
     ]