feat: FontResolver and FontProvider architecture (Block B)

[ralflang]

Create FontProvider interface with CoreFontProvider and TrueTypeFontProvider.

Create a default FontResolver that accepts FontProvider[]|FontProviderList and resolves family+style+weight
to a concrete Font instance.

TrueTypeFontProvider scans a configured directory.

Integrate into PdfWriter: setFont() uses FontResolver when family isn't a core font.
Tests: resolve core font, resolve TTF font, fallback on missing font.