Add BUG-04 test suite + .gitignore for .venv/

Re-holt die Tests vom BUG-04-Branch in diesen Branch, damit CI/Runs
vollständig sind. .gitignore ergänzt um .venv/ und tests/__pycache__/.

tests/test_layout_firstfit.py

@@ -0,0 +1,56 @@
+"""Tests für layout.first_fit (BUG-04).
+
+Behauptung: Add-Item darf bestehende Items NICHT verschieben.
+- Leeres Layout → Add 2×2 hello → Position (0,0), kein Pack
+- Layout mit Lücke → Add 1×1 hello → landet in erster Lücke, andere bleiben
+- Layout voll → Add → None, kein anderes Item verändert
+"""
+import os, sys
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+import unittest
+from layout import Item, first_fit, GRID_COLS, GRID_ROWS
+
+
+class TestFirstFit(unittest.TestCase):
+    def test_first_fit_empty_grid(self):
+        """Auf leerem Grid wird Item bei (0,0) platziert — kein Pack nötig."""
+        new_item = Item("new1", "hello", 0, 0, 1, 1)
+        placed = first_fit(new_item, [])
+        self.assertIsNotNone(placed)
+        self.assertEqual((placed.x, placed.y), (0, 0))
+
+    def test_first_fit_does_not_move_existing(self):
+        """Items mit Lücke: hello wird in Lücke gesetzt, andere bleiben."""
+        existing = [
+            Item("a", "clock",   0, 0, 2, 2),
+            Item("b", "weather", 2, 0, 2, 2),
+            Item("c", "system",  0, 2, 2, 2),
+            Item("d", "spotify", 2, 2, 2, 1),
+        ]
+        before = {(it.id, it.x, it.y) for it in existing}
+        new_item = Item("new", "hello", 0, 0, 1, 1)
+        placed = first_fit(new_item, existing)
+        self.assertIsNotNone(placed)
+        after = {(it.id, it.x, it.y) for it in existing}
+        self.assertEqual(before, after, f"EXISTING MOVED! before={before} after={after}")
+        self.assertEqual((placed.x, placed.y), (2, 3))
+
+    def test_first_fit_full_grid_returns_none(self):
+        """Wenn kein Platz: None zurück, keine Mutation."""
+        existing = [
+            Item("a", "x", 0, 0, 2, 2),
+            Item("b", "y", 2, 0, 2, 2),
+            Item("c", "z", 0, 2, 2, 2),
+            Item("d", "w", 2, 2, 2, 2),
+        ]
+        before = {(it.id, it.x, it.y, it.w, it.h) for it in existing}
+        new_item = Item("new", "hello", 0, 0, 1, 1)
+        placed = first_fit(new_item, existing)
+        self.assertIsNone(placed)
+        after = {(it.id, it.x, it.y, it.w, it.h) for it in existing}
+        self.assertEqual(before, after)
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)