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

programs/include/montauk/syscall.h

@@ -339,6 +339,7 @@ namespace montauk {
     // the buffer to show; the hardware latches it at vblank (tear-free).
     // flags bit0 = block until the flip has been latched (vsync).
     // fb_flip(-1, 0) returns 1 when page flipping is available, 0 when not.
+    // fb_flip(-2, 0) acquires ownership and returns the live buffer (0 or 1).
     inline int64_t fb_flip(int64_t index, uint64_t flags) {
         return syscall2(montauk::abi::SYS_FBFLIP, (uint64_t)index, flags);
     }