fix: address mouse capture review feedback

[Copilot]

Addresses all review comments from the mouse capture PR review.

Description of Changes

NativeMouseCapture.cs

• Renamed x11Display/isCaptured → _x11Display/_isCaptured (underscore-prefix convention)
• Fixed macOS DllImport: "libSystem.dylib" → "CoreGraphics" (symbols live in CoreGraphics framework, not libSystem)
• DisableCapture(): _isCaptured now only set to false after the platform call succeeds — prevents state/OS mismatch on failure, and ensures Cleanup() still releases if an earlier disable failed
• XGrabPointer confineTo: IntPtr.Zero → windowHandle — without this, X11 grab doesn't actually confine the cursor

MainWindow.axaml.cs

• Replaced LayoutUpdated (fires every layout pass → repeated native calls) with SizeChanged/PositionChanged for re-applying capture only on actual bounds changes
• Renamed AttemptCaptureIfPointerInView → AttemptCapture — no pointer-in-view check was ever performed; name was misleading
• Added Deactivated/Activated handlers: capture is released on focus loss (Alt+Tab) and restored on re-activation, preventing the cursor from being trapped outside the window

Rationale behind Changes

The original implementation had several correctness and behavioral issues: internal state could desync from OS state on failed platform calls, X11 capture didn't actually confine the cursor, LayoutUpdated caused excessive native syscalls, and Alt+Tab would leave the cursor permanently trapped.

Suggested Testing Steps

• Toggle capture with middle-click; Alt+Tab away and back — cursor should be released and recaptured automatically
• Resize/move the window while captured — clip rect should update correctly
• On Linux: verify cursor is actually confined to the window boundary
• On macOS: verify no DllNotFoundException at runtime

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