tty + vt terminal split, rsdosedit editor (1:1 UNIX console)

Kernel now has the classic UNIX console split, so full-screen ANSI apps
can run in ring 0:

- vt.rs: terminal emulator owning the text screen — cell grid (char +
  fg/bg), logical cursor (inverse-video), scrolling, and a small ANSI
  CSI parser (cursor position H/f, erase J/K, moves A-D, SGR 16-colour
  with correct ANSI→VGA mapping, cursor show/hide ?25).  Blits only
  changed cells; scroll shifts cell grids + pixels and re-rasterises
  only the exposed bottom row.  gop_println!/clear_screen now render
  through the vt; gop.rs is the raw rasteriser (colored glyph blits,
  pixel shift).
- tty.rs: line discipline — canonical read_line (echo, line buffer,
  ^C cancel, ^D EOF) moved out of the shell, and raw read_byte that
  translates KeyEvents to the terminal byte protocol (arrows → ESC[A-D,
  Backspace → 0x7f, ^X → 0x18, Ctrl+arrows → ESC[1;5A-D, Home/End/PgUp/
  PgDn/Delete).  interrupt_pressed() stays for streaming commands.
- ps2: generic Ctrl(char) + CtrlArrow events (was: only CtrlC/CtrlD).
- rsdosedit: no_std port of the RNG rsedit editor as the first real
  app — undo, syntax highlighting (verified green/gray/yellow on the
  framebuffer), search, word nav, smart indent, ^S save / ^X exit /
  ^X again to discard.  termios → tty raw, stdout → vt, std::fs → VFS
  (ramfs), SIGWINCH → fixed geometry, force-exit window → key counter.
- shell: 'edit <path>' command; read_line/strbuf moved to rsnix.
- CONTEXT.md: layout + milestones + known-issue perf notes.

Verified in QEMU: full edit → type → arrow-move → insert → save → exit
round trip with byte-exact file content; ANSI colors via screendump.

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