TitleBar explicit false close path can corrupt WinUI title-bar teardown

[codemonkeychris]

PR #536 exposed a process crash in the TitleBar_ExplicitFalseOverrides selftest when closing a window whose WindowSpec sets ExtendsContentIntoTitleBar = false while the content still renders a TitleBar element. TitleBarDescriptor already no-ops registration when the value is explicitly false, but Window.Close/host disposal can still hit WinUI title-bar interop teardown and terminate with STATUS_HEAP_CORRUPTION. The PR uses a selftest teardown mitigation (Hide + UnregisterWindowMonitor) so CI can proceed. Follow up with a Reactor-side close/dispose fix or a safer TitleBar unmount path for explicit-false windows.