feat: add native macOS overlay window with pulsing border

Uses NSWindow + PulseBorderView with Core Animation-style pulse.
Fixed NSRect handling from plan: uses NSInsetRect instead of
non-existent .insetBy method on pyobjc tuples.

Made-with: Cursor

scripts/test_overlay.py

@@ -0,0 +1,32 @@
+"""Manual test: shows a pulsing border around Cursor for 10 seconds."""
+import sys
+import time
+
+from Cocoa import NSApplication, NSRunLoop, NSDate
+
+from cursor_flasher.config import Config
+from cursor_flasher.overlay import OverlayWindow
+from cursor_flasher.detector import CursorDetector
+
+app = NSApplication.sharedApplication()
+
+config = Config()
+overlay = OverlayWindow(config)
+detector = CursorDetector()
+
+pid = detector._find_cursor_pid()
+if pid is None:
+    print("Cursor not running")
+    sys.exit(1)
+
+print(f"Showing overlay for PID {pid} for 10 seconds...")
+overlay.show(pid)
+
+end_time = time.time() + 10
+while time.time() < end_time:
+    NSRunLoop.currentRunLoop().runUntilDate_(
+        NSDate.dateWithTimeIntervalSinceNow_(0.1)
+    )
+
+overlay.hide()
+print("Done.")