Initial FocusFixer implementation

Menu-bar utility that fixes cross-display keyboard focus loss: a
listen-only CGEvent tap locates the window under the cursor and
reasserts app activation + AX focus on it, with debounce, tap
auto-recovery, and an Accessibility permission gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

FocusFixerTests/DebounceTests.swift

@@ -0,0 +1,40 @@
+import XCTest
+@testable import FocusFixer
+
+final class DebounceTests: XCTestCase {
+    func testFirstEventIsAlwaysProcessed() {
+        var clockValue: TimeInterval = 0
+        var debouncer = Debouncer(interval: 0.15, clock: { clockValue })
+        XCTAssertTrue(debouncer.shouldProcess())
+    }
+
+    func testEventWithinIntervalIsSuppressed() {
+        var clockValue: TimeInterval = 0
+        var debouncer = Debouncer(interval: 0.15, clock: { clockValue })
+        XCTAssertTrue(debouncer.shouldProcess())
+
+        clockValue += 0.05
+        XCTAssertFalse(debouncer.shouldProcess())
+    }
+
+    func testEventAfterIntervalIsProcessed() {
+        var clockValue: TimeInterval = 0
+        var debouncer = Debouncer(interval: 0.15, clock: { clockValue })
+        XCTAssertTrue(debouncer.shouldProcess())
+
+        clockValue += 0.2
+        XCTAssertTrue(debouncer.shouldProcess())
+    }
+
+    func testAcceptedEventResetsTheWindow() {
+        var clockValue: TimeInterval = 0
+        var debouncer = Debouncer(interval: 0.15, clock: { clockValue })
+        XCTAssertTrue(debouncer.shouldProcess())
+
+        clockValue += 0.2
+        XCTAssertTrue(debouncer.shouldProcess())
+
+        clockValue += 0.05
+        XCTAssertFalse(debouncer.shouldProcess())
+    }
+}