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]>

Cargo.toml

@@ -55,6 +55,9 @@ ttf-parser = "0.25"
 cosmic-text = "0.15"
 unicode-bidi = "0.3"
 lyon_tessellation = "1.0.20"
+# Big-font (.shx) lookups: encode a char to the font's Big5 / GBK / Shift_JIS
+# / EUC-KR double-byte code. Already in the tree through other deps.
+encoding_rs = "0.8"
 pollster = "0.4"
 
 [dev-dependencies]