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, 0 insertions, 1 deletions

diff --git a/shell/src/lib.rs b/shell/src/lib.rs
index ec2474d..4601b98 100644
--- a/shell/src/lib.rs
+++ b/shell/src/lib.rs
@@ -279,7 +279,6 @@ fn cmd_hexdump(args: &str) {
     }
 }
 
-
 // ---------------------------------------------------------------------------
 // edit — run the built-in editor (rsdosedit) on a file
 // ---------------------------------------------------------------------------