fix: correct \boldsymbol rendering for capital Greek and digits

Previously, `\boldsymbol` was aliased to `\mathbf`, producing upright bold for every character. LaTeX convention preserves each character's default italicness when `\boldsymbol` is applied, so Latin letters and lowercase Greek should be bold italic while capital Greek and digits (upright by default) should be bold upright. This PR introduces a `Font::BoldSymbol` variant so `\boldsymbol` can select the correct Unicode math-alphanumeric code point per character class without disturbing `\mathbf` or other font selectors. Fixes #32