classic57 CH excel issue

[shps951023]

[openaitx-system]

Fix: CJK Font Embedding for Chinese Text Rendering

Root Cause: The PDF Unicode font (F2, CIDFontType2) referenced "Arial" but had no embedded font program (/FontFile2). Without embedded font data, PDF viewers cannot render CJK glyphs they show blank boxes or nothing.

Changes made in src/MiniPdf/PdfWriter.cs:

1. System CJK font discovery (FindSystemCjkFont) Locates a CJK-capable TrueType font on the OS:

   • Windows: Microsoft YaHei, SimSun, SimHei, etc.
   • macOS: PingFang, STHeiti, Hiragino Sans GB
   • Linux: Noto Sans CJK, WenQuanYi Micro Hei

2. TrueType font loading (LoadTtfFont, ExtractTtfFromTtc) Reads TrueType/TTC font files and extracts the first font from TTC collections.

3. TrueType table parsing Parses cmap (Unicode glyph ID mapping), hmtx (glyph advance widths), head/OS2/hhea (font metrics) tables.

4. Font subsetting (SubsetTtfFont) Zeros out glyph outlines for unused characters, preserving font structure while enabling excellent compression. Reduces embedded font from ~12 MB to ~280 KB for typical CJK documents.

5. PDF font embedding Added two new PDF objects:

   • Object 8: /FontFile2 compressed subset TrueType font program
   • Object 9: /CIDToGIDMap compressed CID-to-glyph-ID mapping

6. Proper glyph widths (BuildWArray) Generates per-character /W width entries from the font's hmtx table for correct text layout.

Benchmark Results (classic57_cjk_only):

Metric	Value
Overall Score	0.9112
Visual Similarity	0.9953 (99.5%)
Text Similarity	0.7826
PDF Size	337 KB (with embedded font subset)

All 61 Classic tests pass. All 150 xlsx-to-pdf conversions succeed with 0 failures.