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.

1 files changed, 36 insertions, 2 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 66ab320..b557192 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -101,10 +101,20 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
 - `fs.rs` — VFS: dirs, ramfs files, char devices (`/dev/mem`, `rtc`, `pci`,
   `version`, `meminfo`, `null`, `zero`, `console`, `serial`; `/tmp` ramfs;
   `/etc/motd`)
-- `gop.rs` — framebuffer access and text rendering
+- `gop.rs` — framebuffer rasteriser (glyph blits, pixel shifts); the text
+  surface itself lives in `vt`
+- `vt.rs` — terminal emulator: cell grid, cursor, ANSI CSI parser (H/J/K,
+  cursor moves, SGR 16-colour, `?25l/h`), dirty-cell blitting
+- `tty.rs` — line discipline: canonical `read_line` (echo, line buffer,
+  ^C/^D) and raw `read_byte` (terminal byte protocol: arrows → `ESC [ A–D`,
+  ^X → `0x18`, …)
+- `strbuf.rs` — stack fixed-capacity ASCII string (moved from shell)
 - `font.rs` — 8×16 bitmap font data
-- `ps2.rs` — PS/2 keyboard driver (polling)
+- `ps2.rs` — PS/2 keyboard driver (polling); generic `Ctrl(char)` and
+  `CtrlArrow` events
 - `shell.rs` — command loop and built-in commands
+- `rsdosedit/` — kernel-native port of the RNG `rsedit` terminal editor
+  (no_std; talks to `tty` raw mode + `vt` ANSI; saves via the VFS)
 
 ## Milestone status (v0.1)
 
@@ -133,6 +143,30 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
 ✅ **^C / ^D** — Ctrl+C cancels the line or aborts a running stream
   (`cat`/`hexdump` check between chunks), Ctrl+D on an empty line
   powers off (EOF)
+✅ **tty + vt (terminal) — the 1:1 UNIX console split**: `tty` owns the
+  line discipline (canonical echo/buffer/^C/^D for the shell; raw mode
+  translates keys to the classic terminal byte protocol), `vt` owns the
+  screen (cell grid + ANSI CSI parser + 16-colour SGR + dirty-cell blits
+  into the GOP framebuffer).  `gop_println!` etc. now render through `vt`.
+✅ **rsdosedit** — the RNG `rsedit` editor ported to the kernel (no_std,
+  no libc): `edit <path>` opens a full-screen editor with undo, syntax
+  highlighting, search (^W), save (^S), exit (^X, twice to discard), word
+  nav (Ctrl+arrows), smart indent.  Files live in the ramfs.  Termios →
+  `tty` raw mode, stdout → `vt`, `std::fs` → VFS, SIGWINCH → fixed
+  geometry.  Verified in QEMU: edit/type/navigate/insert/save/exit, exact
+  bytes via `cat`/`hexdump`, syntax colours via framebuffer dump.
+
+## Known issues / perf notes
+
+- **Scroll is slow under QEMU without KVM**: each scroll shifts the whole
+  framebuffer up (~3 MB copy at 1280×800) with TCG-emulated stores.
+  Deferred optimisation: ring-buffer rows / deferred pan so scrolling
+  never touches the whole framebuffer.  The vt's cell grid already avoids
+  re-rasterising on scroll (only the exposed bottom row is blitted).
+- **Editor redraw** re-blits changed cells per keystroke (fine at 1280×800);
+  the "optimize drawing someday" item.
+- **ramfs only**: edited files live in memory, lost on reboot until disk
+  support (AHCI/FAT32) lands.
 
 ## Target audience