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, 11 insertions, 7 deletions

diff --git a/rsdos/src/main.rs b/rsdos/src/main.rs
index dc59e8e..c36660e 100644
--- a/rsdos/src/main.rs
+++ b/rsdos/src/main.rs
@@ -83,13 +83,17 @@ pub extern "efiapi" fn efi_main(image_handle: Handle, system_table: *mut SystemT
         rsnix::serial_println!("gop: not found (0x{:x})", gop_result);
     }
     rsnix::vt::init();
-    rsnix::serial_println!("vt: terminal ready ({}x{})", {
-        let (rows, cols) = rsnix::vt::size();
-        rows
-    }, {
-        let (rows, cols) = rsnix::vt::size();
-        cols
-    });
+    rsnix::serial_println!(
+        "vt: terminal ready ({}x{})",
+        {
+            let (rows, cols) = rsnix::vt::size();
+            rows
+        },
+        {
+            let (rows, cols) = rsnix::vt::size();
+            cols
+        }
+    );
 
     // ExitBootServices — we now own the machine
     if unsafe { (bs.exit_boot_services)(image_handle, map_key) } != EFI_SUCCESS {