term: latch SCY in VBlank - fix ring-scroll shear on live displays

The ring-scroll wrote SCY mid-frame; SCY is sampled per scanline (on
hardware and in sl0pboy's PPU), so a scroll landing mid-frame rendered
the top of the frame at the old offset and the bottom at the new one -
a one-frame shear, invisible in frame-sampled GIF captures but ugly on
a live sixel view.

term_view_update now writes hSCY (HRAM) and a transparent VBlank ISR
(push af / apply / pop af / reti, same profile as TimerISR) copies it
to rSCY, so the view only ever moves at frame boundaries. The scroll's
tile+map writes stay immediate: the new map row is invisible at the
old SCY by construction (it's the ring row one past the visible 18).

1 files changed, 18 insertions, 3 deletions

diff --git a/src/boot.asm b/src/boot.asm
index 4049f9d..88eb240 100644
--- a/src/boot.asm
+++ b/src/boot.asm
@@ -15,7 +15,7 @@ SECTION "rst30_syscall", ROM0[$30]
 ; Interrupt vectors
 ; -----------------------------------------------------------------------------
 SECTION "vblank", ROM0[$40]
-    reti
+    jp VBlankISR            ; applies the latched SCY (see hram.asm hSCY)
 SECTION "stat", ROM0[$48]
     reti
 SECTION "timer", ROM0[$50]
@@ -83,7 +83,8 @@ KernelInit:
 
     ; system tick: hardware timer at 16384 Hz input, TMA=0 -> TIMA overflow
     ; IRQ at 64 Hz. TimerISR just counts (wTicks); scheduling stays
-    ; cooperative. Only the timer bit is enabled in IE.
+    ; cooperative. VBlank is enabled too, but its ISR is equally
+    ; transparent: it only applies the latched SCY (terminal ring-scroll).
     xor a
     ld [rTIMA], a
     ld [rTMA], a
@@ -91,7 +92,7 @@ KernelInit:
     ld [rTAC], a
     xor a
     ld [rIF], a
-    ld a, IEF_TIMER
+    ld a, IEF_TIMER | IEF_VBLANK
     ld [rIE], a
     ei
 
@@ -113,3 +114,17 @@ ClearKernelRAM:
     or c
     jr nz, .loop
     ret
+
+; -----------------------------------------------------------------------------
+; VBlankISR - transparent, like TimerISR: applies the latched SCY so terminal
+; ring-scrolls land at frame boundaries. SCY is sampled per scanline, so a
+; mid-frame write shears the picture (top at the old offset, bottom at the
+; new) - exactly the sixel-view glitch this ISR exists to prevent.
+; -----------------------------------------------------------------------------
+SECTION "vblank_isr", ROM0
+VBlankISR:
+    push af
+    ldh a, [hSCY]
+    ldh [$FF42], a              ; rSCY
+    pop af
+    reti