Add basic AppLocator and AXWindowResolver coverage

Tests/AXorcistTests/AXWindowResolverTests.swift

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+import AppKit
+import CoreGraphics
+@testable import AXorcist
+import Testing
+
+@Suite("AXWindowResolver")
+struct AXWindowResolverTests {
+    private let resolver = AXWindowResolver()
+
+    @Test("windowID returns nil for non-window element")
+    @MainActor
+    func windowIdNilForNonWindow() async {
+        let systemWide = AXUIElementCreateSystemWide()
+        let element = Element(systemWide)
+        #expect(self.resolver.windowID(from: element) == nil)
+    }
+
+    @Test("windowExists false for random ID")
+    func windowExistsFalse() {
+        #expect(self.resolver.windowExists(windowID: 999_999_999) == false)
+    }
+}

Tests/AXorcistTests/AppLocatorTests.swift

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+import AppKit
+import CoreGraphics
+@testable import AXorcist
+import Testing
+
+@Suite("AppLocator")
+struct AppLocatorTests {
+    @Test("returns frontmost when point is nil and front app has window under mouse")
+    @MainActor
+    func frontmostPreferred() async throws {
+        // Skip on headless CI where NSEvent.mouseLocation is (0,0) and no frontmost app.
+        guard let front = NSWorkspace.shared.frontmostApplication else { return }
+
+        let app = AppLocator.app(at: nil)
+        // If we have any frontmost app, AppLocator should return something (frontmost fallback).
+        #expect(app != nil)
+        // Best-effort check: if the frontmost app is the only candidate, expect it.
+        #expect(app?.processIdentifier == front.processIdentifier)
+    }
+
+    @Test("falls back to frontmost when no window matches point")
+    @MainActor
+    func fallbackToFrontmost() async throws {
+        guard let front = NSWorkspace.shared.frontmostApplication else { return }
+        // Pick an off-screen point unlikely to hit a window.
+        let offscreen = CGPoint(x: -10_000, y: -10_000)
+        let app = AppLocator.app(at: offscreen)
+        #expect(app?.processIdentifier == front.processIdentifier)
+    }
+}

Tests/AXorcistTests/Support/CommonTestHelpers.swift

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+import Foundation
+
+enum TestError: Error {
+    case invalidInput
+}
+
+extension TimeInterval {
+    static func milliseconds(_ value: Int) -> TimeInterval {
+        TimeInterval(Double(value) / 1000.0)
+    }
+}