Implement FontKind enum and classify_font() function for Phase 2.1
font type detection. Includes strip_subset_prefix() for handling
font subset names (e.g., ABCDEF+Times-Roman).
FontKind variants:
- Type1, Type1Std14 (Standard 14)
- TrueType, OpenTypeCFF
- Type0, CIDFontType0, CIDFontType2
- Type3
Classifier reads /Subtype, /BaseFont, and for Type0 fonts, descendant
CIDFont subtype. OpenTypeCFF detected via /FontDescriptor /FontFile3
with /Subtype /OpenType.
All 27 font tests pass.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
