Ignore SDL3 mouse delta while window is unfocused

[Copilot]

SDL3 mouse motion events can continue arriving after the window loses focus on Windows, which allows hover movement over an unfocused window to generate camera delta. This change gates mouse motion on window focus and re-syncs mouse state when focus returns.

• Focus-aware mouse motion handling

  • Track SDL window focus inside SdlMouseInput
  • Drop SDL_EVENT_MOUSE_MOTION events while the window is unfocused
  • Clear queued motion events on focus loss so stale deltas are not delivered later

• State resync on focus regain

  • Refresh window metrics and reinitialize the current mouse position on SDL_EVENT_WINDOW_FOCUS_GAINED
  • Preserve existing cursor visibility / relative-mode behavior while preventing the first post-focus event from using stale coordinates

• Regression coverage

  • Add a focused unit test that verifies mouse motion is ignored when the SDL window is marked unfocused

if (event.motion().windowID() != context.getWindowId() || !windowFocused) {
    return;
}

[riccardobl]

looks good