vt: decode UTF-8, render box-drawing/arrow glyphs procedurally

The editor's gutter '│' (U+2502) appeared as '???' — write_bytes fed
each UTF-8 byte to the state machine, which mapped every byte >= 0x80
to '?', and the Latin-1 font has no box-drawing glyphs anyway.

- vt.write_bytes now decodes UTF-8 sequences; printable non-ASCII
  dispatches to the glyph path instead of the byte parser.
- gop: unicode_glyph_bits() procedurally rasterises box-drawing
  (U+2500-257F light/heavy/double), arrows (U+2190-2193) and the em
  dash (U+2014) — pure geometry, no font data needed; blit_glyph falls
  back to it for ch >= 256.

Verified via framebuffer dumps: editor gutter │ renders as a vertical
line, help screen ═ separators and ←→↑↓ all render as lines (no '?').
Shell banner em dash renders too.

1 files changed, 3 insertions, 1 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index b557192..20c60f7 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -104,7 +104,9 @@ Single Rust crate (`x86_64-unknown-uefi` target).  Module breakdown:
 - `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
+  cursor moves, SGR 16-colour, `?25l/h`), dirty-cell blitting, UTF-8
+  decoding (box-drawing `│`/`═`, arrows `←↑→↓`, em dash `—` render
+  procedurally instead of `?`)
 - `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`, …)