feat: vblank IRQ and double-buffered page flip for Intel GPU

Add tear-free scanout to the Intel GPU driver: a second kernel-allocated
scanout buffer, DSPASURF flips latched at vblank, and a vblank interrupt
delivered over MSI (Gen 11+ master/display/pipe IRQ chain) with a
monotonic vblank counter and WaitVblank().

Expose it as SYS_FBFLIP (150): index selects the front buffer, -1
queries support, flags bit0 waits for the flip to latch. fb_map() now
maps buffer 1 right after buffer 0 when flipping is available, and
gui::Framebuffer draws to the off-screen buffer and flips with vsync,
falling back to the direct copy when unsupported.

Scanout is restored to buffer 0 when the flip-owning process exits and
on panic, so the terminal and panic box never land on the invisible
buffer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

kernel/src/Api/Syscall.cpp

@@ -148,6 +148,8 @@ namespace montauk::abi {
                 return 0;
             case SYS_FBMAP:
                 return (int64_t)Sys_FbMap();
+            case SYS_FBFLIP:
+                return Sys_FbFlip(frame->arg1, frame->arg2);
             case SYS_TERMSIZE:
                 return (int64_t)Sys_TermSize();
             case SYS_GETARGS: