fix(shx): unifont subshapes are two-byte codes

Since 2026.39 reads `unifont 1.0` files (isocp.shx and its family among
them) instead of falling back to a substitute, a glyph built from a
subshape went wrong: code 7 is followed by a two-byte code point in a
unifont, but only big fonts read two bytes, so the low byte ran on as a
drawing opcode. ISOCP's `9` came out at x-height as a small bowl.

Fixes #1556

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/text/shx.rs

@@ -544,16 +544,26 @@ fn interpret(
                 }
             }
             7 => {
-                let mut sub = *b.get(i).unwrap_or(&0) as u16;
-                i += 1;
-                // A big font references subshapes by a two-byte code:
-                // `7, 0, hi, lo`.
-                if file.kind == ShxKind::Bigfont && sub == 0 {
-                    let hi = *b.get(i).unwrap_or(&0) as u16;
-                    let lo = *b.get(i + 1).unwrap_or(&0) as u16;
+                let two_bytes = |i: usize| {
+                    ((*b.get(i).unwrap_or(&0) as u16) << 8) | *b.get(i + 1).unwrap_or(&0) as u16
+                };
+                // A unifont names its subshape by a two-byte code point
+                // (`7, hi, lo`); read as one byte, the low byte ran on as an
+                // opcode and the glyph came out mangled (ISOCP's `9`, #1556).
+                // A big font uses `7, 0, hi, lo`; a shape file one byte.
+                let sub = if file.kind == ShxKind::Unifont {
                     i += 2;
-                    sub = (hi << 8) | lo;
-                }
+                    two_bytes(i - 2)
+                } else {
+                    let sub = *b.get(i).unwrap_or(&0) as u16;
+                    i += 1;
+                    if file.kind == ShxKind::Bigfont && sub == 0 {
+                        i += 2;
+                        two_bytes(i - 2)
+                    } else {
+                        sub
+                    }
+                };
                 if !skipping && sub != 0 && sub != shape_number {
                     interpret(file, sub, st, cur, out, depth + 1);
                 }