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) <noreply@anthropic.com>

src/entities/text_support.rs

@@ -77,6 +77,27 @@ pub fn resolve_text_style(style_name: &str, document: &CadDocument) -> ResolvedT
         }
     }
 
+    // A style's "Big Font" (`chineset.shx`, `hztxt.shx`, …) supplies the
+    // double-byte characters. Resolve it like the primary font (next to the
+    // drawing, then the per-user fonts folder) and pair it with the face; an
+    // unresolvable big font just leaves those characters to the fallback.
+    if let Some(big) = style
+        .map(|s| s.big_font_file.trim())
+        .filter(|big| !big.is_empty() && big.to_ascii_lowercase().ends_with(".shx"))
+    {
+        let base = document
+            .source_path
+            .as_deref()
+            .map(std::path::Path::new)
+            .and_then(|p| p.parent());
+        let big_path = crate::io::resolve_image_file(big, base).or_else(|| {
+            crate::io::font_repo::local_font_file(big).map(|p| p.to_string_lossy().into_owned())
+        });
+        if let Some(big_path) = big_path {
+            font_name = Face::with_big_font(&font_name, &big_path);
+        }
+    }
+
     ResolvedTextStyle {
         font_name,
         width_factor: style.map(|s| s.width_factor as f32).unwrap_or(1.0),