experiment(vegas): vegas_scroll.switch_interval_ms shortens the GIL switch interval during a run

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_vegas_switch_interval.py

@@ -0,0 +1,41 @@
+"""vegas_scroll.switch_interval_ms: applied for a Vegas run, restored after."""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from src.vegas_mode.config import VegasModeConfig  # noqa: E402
+from src.vegas_mode.coordinator import VegasModeCoordinator  # noqa: E402
+
+
+def _coordinator(ms):
+    c = VegasModeCoordinator.__new__(VegasModeCoordinator)
+    c.vegas_config = VegasModeConfig(switch_interval_ms=ms)
+    return c
+
+
+def test_applied_for_the_run_and_restored_after():
+    before = sys.getswitchinterval()
+    c = _coordinator(1.0)
+    try:
+        c._apply_switch_interval()
+        assert abs(sys.getswitchinterval() - 0.001) < 1e-9
+        c._apply_switch_interval()      # a second start must not lose the original
+    finally:
+        c._restore_switch_interval()
+    assert sys.getswitchinterval() == before
+
+
+def test_zero_leaves_the_interpreter_alone():
+    before = sys.getswitchinterval()
+    c = _coordinator(0.0)
+    c._apply_switch_interval()
+    c._restore_switch_interval()
+    assert sys.getswitchinterval() == before
+
+
+def test_read_from_config():
+    config = VegasModeConfig.from_config(
+        {"display": {"vegas_scroll": {"switch_interval_ms": 1}}})
+    assert config.switch_interval_ms == 1.0
+    assert VegasModeConfig.from_config({"display": {"vegas_scroll": {}}}).switch_interval_ms == 0.0