lint: ruff cleanup pass across bal/ and tests/

- Sort imports and fix pyproject ruff config (per-file ignores for
  intentional Qt/core exceptions)
- Mark Heirs.validate_* helpers as @staticmethod
- Clean up dead code, rename shadowing vars, use raise ... from
- Add AGENTS.md with env/lint/test/release guidance

tests/preview_wizard_settings_align.py

@@ -27,12 +27,19 @@ import os
 
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
-from PyQt6.QtWidgets import (  # noqa: E402
-    QApplication, QWidget, QVBoxLayout, QHBoxLayout, QToolButton, QComboBox,
-    QLineEdit, QSpinBox, QLabel,
-)
-from PyQt6.QtGui import QFontMetrics  # noqa: E402
 from PyQt6.QtCore import Qt  # noqa: E402
+from PyQt6.QtGui import QFontMetrics  # noqa: E402
+from PyQt6.QtWidgets import (  # noqa: E402
+    QApplication,
+    QComboBox,
+    QHBoxLayout,
+    QLabel,
+    QLineEdit,
+    QSpinBox,
+    QToolButton,
+    QVBoxLayout,
+    QWidget,
+)
 
 
 def _char_w():