fix: kernel concurrency, interrupt context, and user VA safety

kernel/src/Common/Panic.cpp

@@ -70,6 +70,7 @@ void Panic(const char *meditationString, System::PanicFrame* frame) {
     // showing the other buffer, the panic box would land on an invisible
     // surface. Safe no-op when the GPU driver is inactive.
     Drivers::Graphics::IntelGPU::PanicRestoreScanout();
+    Kt::EnablePanicOutput();
 
     // Header
     kerr << BOXUI_ANSI_RED_BG << BOXUI_ANSI_WHITE_FG << BOXUI_ANSI_BOLD << "\n";
@@ -105,5 +106,11 @@ void Panic(const char *meditationString, System::PanicFrame* frame) {
     PrintHorizontalEdge(BOXUI_BL, BOXUI_BR);
     kerr << BOXUI_ANSI_RESET;
 
+#if defined (__x86_64__)
+    // The framebuffer is write-combining. Drain the final panic text before
+    // halting all forward progress so it cannot remain in a CPU write buffer.
+    asm volatile("sfence" ::: "memory");
+#endif
+
     Halt();
 }