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, 20 insertions, 4 deletions

diff --git a/rsnix/src/ps2.rs b/rsnix/src/ps2.rs
index 8dd58a0..4747813 100644
--- a/rsnix/src/ps2.rs
+++ b/rsnix/src/ps2.rs
@@ -337,10 +337,26 @@ fn process_e0(byte: u8) -> Option<KeyEvent> {
     update_modifiers(byte, true, true);
 
     match byte {
-        0x48 => Some(if ctrl_held() { KeyEvent::CtrlArrow(0) } else { KeyEvent::ArrowUp }),
-        0x50 => Some(if ctrl_held() { KeyEvent::CtrlArrow(1) } else { KeyEvent::ArrowDown }),
-        0x4B => Some(if ctrl_held() { KeyEvent::CtrlArrow(2) } else { KeyEvent::ArrowLeft }),
-        0x4D => Some(if ctrl_held() { KeyEvent::CtrlArrow(3) } else { KeyEvent::ArrowRight }),
+        0x48 => Some(if ctrl_held() {
+            KeyEvent::CtrlArrow(0)
+        } else {
+            KeyEvent::ArrowUp
+        }),
+        0x50 => Some(if ctrl_held() {
+            KeyEvent::CtrlArrow(1)
+        } else {
+            KeyEvent::ArrowDown
+        }),
+        0x4B => Some(if ctrl_held() {
+            KeyEvent::CtrlArrow(2)
+        } else {
+            KeyEvent::ArrowLeft
+        }),
+        0x4D => Some(if ctrl_held() {
+            KeyEvent::CtrlArrow(3)
+        } else {
+            KeyEvent::ArrowRight
+        }),
         0x47 => Some(KeyEvent::Home),
         0x4F => Some(KeyEvent::End),
         0x49 => Some(KeyEvent::PageUp),