MIL-013: Debian container and Windows Sandbox checks

A Debian image (python:3.13-slim-bookworm with Xvfb and GNOME's window
manager) runs the linters, the whole test suite, the screenshots and the
smoke test of a PyInstaller Linux binary and of the source program, so no
window opens on the development machine. The screenshot tool works on
Linux, a smoke driver presses the known button with the virtual mouse, and
Windows Sandbox scripts are written (not run, they open a window on the
host). README and the wiki Install pages describe it. Review records
RC-077 (Python) and RC-078 (shell).

Task: MIL-013#2
Task: MIL-013#3
Task: MIL-013#4
Task: MIL-013#5
Task: MIL-013#6
Task: MIL-013#7
Task: MIL-013#9
Task: MIL-013#10

tests/unit/test_capture_screenshots.py

@@ -0,0 +1,41 @@
+"""Tests of the pure parts of the screenshot tool.
+
+@file test_capture_screenshots.py
+@brief Checks platform_kind and window_box of tools/capture_screenshots.py.
+"""
+
+import importlib.util
+from pathlib import Path
+from types import ModuleType
+
+import pytest
+
+pytest.importorskip("PIL")
+
+_TOOL_PATH = Path(__file__).resolve().parents[2] / "tools" / "capture_screenshots.py"
+
+
+def _load_tool() -> ModuleType:
+    spec = importlib.util.spec_from_file_location("capture_screenshots", _TOOL_PATH)
+    assert spec is not None
+    assert spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_platform_kind_is_windows_for_win32() -> None:
+    assert _load_tool().platform_kind("win32") == "windows"
+
+
+def test_platform_kind_is_x11_for_linux() -> None:
+    assert _load_tool().platform_kind("linux") == "x11"
+
+
+def test_window_box_adds_menu_height_on_x11() -> None:
+    assert _load_tool().window_box("x11", 10, 100, 900, 726, 28) == (10, 72, 910, 826)
+
+
+def test_window_box_ignores_menu_height_on_windows() -> None:
+    box = _load_tool().window_box("windows", 10, 100, 900, 726, 28)
+    assert box == (10, 100, 910, 826)