vt: defer scrolls and batch framebuffer pushes

Streaming output scrolled one framebuffer push per line (each a
~3-8 MB write-only copy).  The vt now scrolls the logical grid
instantly in RAM and defers the display shift: N pending scrolls are
pushed together as one shift_up_n — one push regardless of batch size.

- vt: PENDING_SCROLLS counter; scroll_up shifts CELLS only; flush
  renders nothing while pending (the diff would otherwise re-rasterise
  the whole screen per line); sync_scroll() does one gop::shift_up_n +
  RENDERED mirror + normal diff flush (which re-blits just the newly
  exposed rows and the cursor).
- Sync triggers: batch of 16 rows, ~40 ms of stream pause (100 Hz
  TICKS), any CSI sequence (full-screen apps like the editor render
  immediately), clear_all/set_cursor_visible, interactive input
  (tty read_line/read_byte), and the shell before each prompt.
- gop: shift_up → shift_up_n(n) (one shadow memcpy + one push).

Verified in QEMU: mmap/hexdump streaming renders in coherent batches
with the prompt syncing at the end; clear wipes clean; the editor
scrolls and redraws immediately (45-line file, gutter + status bar
intact, cursor correct); save/exit/cat round-trip unchanged.  Release
and dev builds pass.

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