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 <[email protected]>

programs/include/Api/Syscall.hpp

@@ -203,6 +203,9 @@ namespace montauk::abi {
     // CPU power/thermal status
     static constexpr uint64_t SYS_POWERINFO           = 149;  // (PowerInfo*) -> 0, -1 unsupported
 
+    // Framebuffer page flip (double-buffered scanout)
+    static constexpr uint64_t SYS_FBFLIP              = 150;  // (index, flags) -> new front index; index=-1 queries support (1/0); flags bit0 = wait vsync
+
     // Tunable parameters (for SYS_SDR_SETPARAM / SYS_SDR_GETPARAM).
     static constexpr int SDR_PARAM_FREQ        = 0;  // center frequency, Hz
     static constexpr int SDR_PARAM_SAMPLE_RATE = 1;  // sample rate, Hz