feat(text): read unifont and big-font .shx files; pair a style's big font

`shx.rs` only accepted the classic `AutoCAD-86 shapes 1.x` container, so
every modern Autodesk font (`txt.shx`, `simplex.shx`, `romans.shx`, … are
`unifont 1.0`) and every Asian big font (`chineset.shx`, `hztxt.shx`,
`bigfont.shx` — `bigfont 1.0`) was rejected and text fell back to the LFF
substitutes and TrueType fallback glyphs. Drawings authored with SHX big
fonts (the norm for Chinese / Japanese / Korean sheets) never showed their
real glyphs.

- Parse all three containers. Unifont glyphs are keyed by Unicode; big-font
  glyphs by their double-byte code from a sparse hash table of
  (code, length, offset) entries. Big fonts reference subshapes with a
  two-byte `7, 0, hi, lo`.
- `bigfont_glyph` encodes a character with encoding_rs (Big5 / GBK /
  Shift_JIS / EUC-KR, most likely first for the file name) and takes the
  first code the font has.
- `Face::WithBig` pairs a style's big font with its primary face: full-width
  characters come from the big font, everything else from the primary —
  how a STYLE with a "Big Font" behaves in AutoCAD. `resolve_text_style`
  resolves `big_font_file` like the primary (next to the drawing, then the
  user fonts folder) and rides it along in the font name.
- The missing-fonts dialog lists unresolvable big fonts too.

Unit tests cover a synthetic big font (Big5 lookup, cap-height scaling, no
aliasing of unknown codes), a synthetic unifont, and the pairing.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/io/font_repo.rs

@@ -70,18 +70,21 @@ pub fn missing_shx_fonts(doc: &CadDocument) -> Vec<String> {
         .map(Path::new)
         .and_then(|p| p.parent());
     for style in doc.text_styles.iter() {
-        let file = style.font_file.trim();
-        if file.is_empty() || !file.to_ascii_lowercase().ends_with(".shx") {
-            continue;
-        }
-        let resolved = crate::io::resolve_image_file(file, base).is_some()
-            || local_font_file(file).is_some();
-        if resolved {
-            continue;
-        }
-        let name = file.rsplit(['/', '\\']).next().unwrap_or(file).to_string();
-        if !missing.iter().any(|m| m.eq_ignore_ascii_case(&name)) {
-            missing.push(name);
+        // The primary font and the big font (Asian double-byte glyphs) are
+        // both plain .shx files the style depends on.
+        for file in [style.font_file.trim(), style.big_font_file.trim()] {
+            if file.is_empty() || !file.to_ascii_lowercase().ends_with(".shx") {
+                continue;
+            }
+            let resolved = crate::io::resolve_image_file(file, base).is_some()
+                || local_font_file(file).is_some();
+            if resolved {
+                continue;
+            }
+            let name = file.rsplit(['/', '\\']).next().unwrap_or(file).to_string();
+            if !missing.iter().any(|m| m.eq_ignore_ascii_case(&name)) {
+                missing.push(name);
+            }
         }
     }
     missing.sort_by_key(|name| name.to_ascii_lowercase());