feat(port): mute on window focus loss

[Zorkats]

What

One independent behavior, off-by-default-friendly and independently toggled from Settings > Audio > Focus Behavior:

Setting	CVar	Default
Mute on Focus Loss	gSettings.FocusControl.MuteOnFocusLoss	Off

Mute: when the window loses focus and the CVar is enabled, gSettings.Audio.MasterVolume is saved and set to 0.0f. On refocus, the saved value is restored. Volume-save state uses a sentinel of -1.0f so a double focus-loss doesn't corrupt the saved value.

Implementation

• port/focus.h / port/focus.cpp — ssb64::RegisterFocusListener() registers a WindowFocusEvent listener via REGISTER_LISTENER. No global listener object; the listener ID is fire-and-forget (unregistered on shutdown via the event system teardown).
• port/hooks/Events.h — adds #include "ship/events/WindowEvents.h" so WindowFocusEvent is available to all port TUs that include Events.h.
• port/hooks/Events.cpp — adds REGISTER_EVENT(WindowFocusEvent) to PortRegisterEvents().
• port/port.cpp — calls ssb64::RegisterFocusListener() immediately after PortRegisterEvents().
• port/gui/PortMenu.cpp — adds "Focus Behavior" separator and one checkbox in the Audio sidebar, after the voice volume slider.

focus.cpp is picked up by file(GLOB_RECURSE SSB64_SRC_PORT "port/*.cpp" ...) in CMakeLists.txt; no CMake edit was needed.

Depends on PR#7 of libultraship.

Test plan

• Build succeeds on Windows (MSVC/Ninja) and Linux (CachyOS, Linux GCC 16.1.1)
• "Mute on Focus Loss" checked: audio silences on alt-tab, restores on refocus
• "Mute on Focus Loss" unchecked: audio unaffected by alt-tab
• Rapid alt-tab (focus-lost/gained in quick succession): volume sentinel prevents double-save corruption
• Settings > Audio shows "Focus Behavior" section with checkbox and tooltip.