feat: introduce a 'code' mode to display source code lines, add new font assets, and include dedicated tests for code fetching.

tests/test_fetch_code.py

@@ -0,0 +1,35 @@
+import re
+
+from engine.fetch_code import fetch_code
+
+
+def test_return_shape():
+    items, line_count, ignored = fetch_code()
+    assert isinstance(items, list)
+    assert line_count == len(items)
+    assert ignored == 0
+
+
+def test_items_are_tuples():
+    items, _, _ = fetch_code()
+    assert items, "expected at least one code line"
+    for item in items:
+        assert isinstance(item, tuple) and len(item) == 3
+        text, src, ts = item
+        assert isinstance(text, str)
+        assert isinstance(src, str)
+        assert isinstance(ts, str)
+
+
+def test_blank_and_comment_lines_excluded():
+    items, _, _ = fetch_code()
+    for text, _, _ in items:
+        assert text.strip(), "blank line should have been filtered"
+        assert not text.strip().startswith("#"), "comment line should have been filtered"
+
+
+def test_module_path_format():
+    items, _, _ = fetch_code()
+    pattern = re.compile(r"^engine\.\w+$")
+    for _, _, ts in items:
+        assert pattern.match(ts), f"unexpected module path: {ts!r}"