macos: window-position-x/y are from top-left corner

Fixes #8672

Almost fully written by AI: https://ampcode.com/threads/T-86df68a3-578c-4a1c-91f3-788f8b8f0aae

I reviewed all the code.

macos/Tests/NSScreenTests.swift

@@ -0,0 +1,99 @@
+//
+//  WindowPositionTests.swift
+//  GhosttyTests
+//
+//  Tests for window positioning coordinate conversion functionality.
+//
+
+import Testing
+import AppKit
+@testable import Ghostty
+
+struct NSScreenExtensionTests {
+    /// Test positive coordinate conversion from top-left to bottom-left
+    @Test func testPositiveCoordinateConversion() async throws {
+        // Mock screen with 1000x800 visible frame starting at (0, 100)
+        let mockScreenFrame = NSRect(x: 0, y: 100, width: 1000, height: 800)
+        let mockScreen = MockNSScreen(visibleFrame: mockScreenFrame)
+        
+        // Mock window size
+        let windowSize = CGSize(width: 400, height: 300)
+        
+        // Test top-left positioning: x=15, y=15
+        let origin = mockScreen.origin(
+            fromTopLeftOffsetX: 15,
+            offsetY: 15,
+            windowSize: windowSize)
+        
+        // Expected: x = 0 + 15 = 15, y = (100 + 800) - 15 - 300 = 585
+        #expect(origin.x == 15)
+        #expect(origin.y == 585)
+    }
+    
+    /// Test zero coordinates (exact top-left corner)
+    @Test func testZeroCoordinates() async throws {
+        let mockScreenFrame = NSRect(x: 0, y: 100, width: 1000, height: 800)
+        let mockScreen = MockNSScreen(visibleFrame: mockScreenFrame)
+        let windowSize = CGSize(width: 400, height: 300)
+        
+        let origin = mockScreen.origin(
+            fromTopLeftOffsetX: 0,
+            offsetY: 0,
+            windowSize: windowSize)
+        
+        // Expected: x = 0, y = (100 + 800) - 0 - 300 = 600
+        #expect(origin.x == 0)
+        #expect(origin.y == 600)
+    }
+    
+    /// Test with offset screen (not starting at origin)
+    @Test func testOffsetScreen() async throws {
+        // Secondary monitor at position (1440, 0) with 1920x1080 resolution
+        let mockScreenFrame = NSRect(x: 1440, y: 0, width: 1920, height: 1080)
+        let mockScreen = MockNSScreen(visibleFrame: mockScreenFrame)
+        let windowSize = CGSize(width: 600, height: 400)
+        
+        let origin = mockScreen.origin(
+            fromTopLeftOffsetX: 100,
+            offsetY: 50,
+            windowSize: windowSize)
+        
+        // Expected: x = 1440 + 100 = 1540, y = (0 + 1080) - 50 - 400 = 630
+        #expect(origin.x == 1540)
+        #expect(origin.y == 630)
+    }
+    
+    /// Test large coordinates
+    @Test func testLargeCoordinates() async throws {
+        let mockScreenFrame = NSRect(x: 0, y: 0, width: 1920, height: 1080)
+        let mockScreen = MockNSScreen(visibleFrame: mockScreenFrame)
+        let windowSize = CGSize(width: 400, height: 300)
+        
+        let origin = mockScreen.origin(
+            fromTopLeftOffsetX: 500,
+            offsetY: 200,
+            windowSize: windowSize)
+        
+        // Expected: x = 0 + 500 = 500, y = (0 + 1080) - 200 - 300 = 580
+        #expect(origin.x == 500)
+        #expect(origin.y == 580)
+    }
+}
+
+/// Mock NSScreen class for testing coordinate conversion
+private class MockNSScreen: NSScreen {
+    private let mockVisibleFrame: NSRect
+    
+    init(visibleFrame: NSRect) {
+        self.mockVisibleFrame = visibleFrame
+        super.init()
+    }
+    
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+    
+    override var visibleFrame: NSRect {
+        return mockVisibleFrame
+    }
+}