feat: Add mcrfpy.step() and synchronous screenshot for headless mode (closes #153)

Implements Python-controlled simulation advancement for headless mode:

- Add mcrfpy.step(dt) to advance simulation by dt seconds
- step(None) advances to next scheduled event (timer/animation)
- Timers use simulation_time in headless mode for deterministic behavior
- automation.screenshot() now renders synchronously in headless mode
  (captures current state, not previous frame)

This enables LLM agent orchestration (#156) by allowing:
- Set perspective, take screenshot, query LLM - all synchronous
- Deterministic simulation control without frame timing issues
- Event-driven advancement with step(None)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/unit/test_synchronous_screenshot.py

@@ -0,0 +1,133 @@
+#!/usr/bin/env python3
+"""
+Test synchronous screenshot in headless mode (#153)
+====================================================
+
+Tests that automation.screenshot() captures the CURRENT state in headless mode,
+not the previous frame's buffer.
+
+Key behavior:
+- In headless mode, screenshot() renders then captures (synchronous)
+- Changes made before screenshot() are visible in the captured image
+- No timer dance required to capture current state
+"""
+
+import mcrfpy
+from mcrfpy import automation
+import sys
+import os
+
+def run_tests():
+    """Run synchronous screenshot tests"""
+    print("=== Synchronous Screenshot Tests ===\n")
+
+    # Create a test scene with UI elements
+    mcrfpy.createScene("screenshot_test")
+    mcrfpy.setScene("screenshot_test")
+    ui = mcrfpy.sceneUI("screenshot_test")
+
+    # Test 1: Basic screenshot works
+    print("Test 1: Basic screenshot functionality")
+    test_file = "/tmp/test_screenshot_basic.png"
+    if os.path.exists(test_file):
+        os.remove(test_file)
+
+    result = automation.screenshot(test_file)
+    assert result == True, f"screenshot() should return True, got {result}"
+    assert os.path.exists(test_file), "Screenshot file should exist"
+    file_size = os.path.getsize(test_file)
+    assert file_size > 0, "Screenshot file should not be empty"
+    print(f"  Screenshot saved: {test_file} ({file_size} bytes)")
+    print()
+
+    # Test 2: Screenshot captures current state (not previous frame)
+    print("Test 2: Screenshot captures current state immediately")
+
+    # Add a visible frame
+    frame1 = mcrfpy.Frame(pos=(100, 100), size=(200, 200))
+    frame1.fill_color = mcrfpy.Color(255, 0, 0)  # Red
+    ui.append(frame1)
+
+    # Take screenshot immediately - should show the red frame
+    test_file2 = "/tmp/test_screenshot_state1.png"
+    if os.path.exists(test_file2):
+        os.remove(test_file2)
+
+    result = automation.screenshot(test_file2)
+    assert result == True, "screenshot() should return True"
+    assert os.path.exists(test_file2), "Screenshot file should exist"
+    print(f"  Screenshot with red frame: {test_file2}")
+
+    # Modify the frame color
+    frame1.fill_color = mcrfpy.Color(0, 255, 0)  # Green
+
+    # Take another screenshot - should show green, not red
+    test_file3 = "/tmp/test_screenshot_state2.png"
+    if os.path.exists(test_file3):
+        os.remove(test_file3)
+
+    result = automation.screenshot(test_file3)
+    assert result == True, "screenshot() should return True"
+    assert os.path.exists(test_file3), "Screenshot file should exist"
+    print(f"  Screenshot with green frame: {test_file3}")
+    print()
+
+    # Test 3: Multiple screenshots in succession
+    print("Test 3: Multiple screenshots in succession")
+    screenshot_files = []
+    for i in range(3):
+        frame1.fill_color = mcrfpy.Color(i * 80, i * 80, i * 80)  # Varying gray
+        test_file_n = f"/tmp/test_screenshot_seq{i}.png"
+        if os.path.exists(test_file_n):
+            os.remove(test_file_n)
+
+        result = automation.screenshot(test_file_n)
+        assert result == True, f"screenshot() {i} should return True"
+        assert os.path.exists(test_file_n), f"Screenshot {i} should exist"
+        screenshot_files.append(test_file_n)
+
+    print(f"  Created {len(screenshot_files)} sequential screenshots")
+
+    # Verify all files are different sizes or exist
+    sizes = [os.path.getsize(f) for f in screenshot_files]
+    print(f"  File sizes: {sizes}")
+    print()
+
+    # Test 4: Screenshot after step()
+    print("Test 4: Screenshot works correctly after step()")
+    mcrfpy.step(0.1)  # Advance simulation
+
+    test_file4 = "/tmp/test_screenshot_after_step.png"
+    if os.path.exists(test_file4):
+        os.remove(test_file4)
+
+    result = automation.screenshot(test_file4)
+    assert result == True, "screenshot() after step() should return True"
+    assert os.path.exists(test_file4), "Screenshot after step() should exist"
+    print(f"  Screenshot after step(): {test_file4}")
+    print()
+
+    # Clean up test files
+    print("Cleaning up test files...")
+    for f in [test_file, test_file2, test_file3, test_file4] + screenshot_files:
+        if os.path.exists(f):
+            os.remove(f)
+
+    print()
+    print("=== All Synchronous Screenshot Tests Passed! ===")
+    return True
+
+# Main execution
+if __name__ == "__main__":
+    try:
+        if run_tests():
+            print("\nPASS")
+            sys.exit(0)
+        else:
+            print("\nFAIL")
+            sys.exit(1)
+    except Exception as e:
+        print(f"\nFAIL: {e}")
+        import traceback
+        traceback.print_exc()
+        sys.exit(1)