fix: implement VGA text-mode scrolling instead of wrap-to-row-0

fix: implement VGA text-mode scrolling instead of wrap-to-row-0

When the cursor reached the bottom of the 25-row VGA text buffer,
term_row was reset to 0, overwriting the top of the screen. This
made boot output unreadable once it exceeded 25 lines.

Add vga_scroll() that shifts all rows up by one and clears the
last row. Called from both vga_put_char and vga_print.

Passes: make, cppcheck, QEMU smoke test.