term: SCY ring-scroll - scroll writes ONE map row; OSK toggle is 212 cycles

The BG map's 32 rows become a ring: logical row L always lives at map
row (wMapTop + L) & 31, and SCY = (wMapTop + wViewTop)*8 places the
view. Scrolling bumps wMapTop, rebuilds the one new bottom line and
writes its single map row (map_row_one); the other 17 rows move for
free. The whole OSK view dance - shift the cursor above the keys,
restore the backlog on hide - is now term_view_update: one SCY write,
no map rewrite at all.

OSK safety: the keyboard lives on the WINDOW layer, which SCY never
moves. Stale ring rows can only appear under it: wViewTop = max(0,
wCurRow-14) <= 3 is nonzero only while the OSK is visible, and the
window covers exactly those bottom 3 rows (WX=7, WY=120). Verified:
scroll + view shift + scroll-while-OSK-up + backlog restore all
pixel-correct via VRAM screenshots; full README demo passes.

term_scroll: 78.5k -> 35.6k T-cycles (857k pre-optimization: 24x).
OSK toggle: 45k tilemap rewrite -> 212 cycles.
Regenerated demo/gbos-demo.gif on the new renderer.

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