[shupphar]

The 0.100 changelog says that \boldsymbol and \bm "preserve the style of the characters, so they remain italic if they were italic before", while \mathbf renders upright. In practice, \bm{x} and \boldsymbol{x} are drawn as an upright bold x (class ML__mathbf) and serialized as \mathbf{x}, so a field round-trips \bm{x}^\top \mathbf{W} \bm{x} to \mathbf{x}^{\top}\mathbf{Wx}.

Cause. \mathbf sets the normal variant plus the bold variant style. \bm and \boldsymbol set only the bold variant style. That difference survives in the atom's style, but both the atom's createBox() and Box defaulted a missing variant to normal before MathMode.getFont() ran, so the two looked identical there. getFont() also only auto-italicized a letter that had no variant style at all.

Fix.

• Rendering. Box and createBox() no longer fill in the variant. getFont() treats a missing variant as normal itself. For a bold with no variant, it applies the same letter shape as for a regular letter: under tex a Latin letter or a lowercase Greek letter becomes italic (Math-BoldItalic, ML__mathbfit), while uppercase Greek and digits stay upright.
• Serialization. emitBoldRun() splits a bold run by how its weight is spelt:
  • An upright bold, where a variant was set, is \mathbf when it is letters and digits, as before.
  • A bold with the automatic shape is \bm, the fallback MathLive already uses, unless it is only digits.
  • A bold italic variant (\mathbfit) carries its own weight, so \mathbfit{x} is no longer written \mathbf{\mathbfit{x}}.

Tests. A BOLD ITALIC block in test/latex.test.ts covers the round trips above, and checks that \bm{x} and \boldsymbol{x} draw with ML__mathbfit, while \mathbf{x} and \bm{\Gamma} don't. The existing suites pass unchanged.

Suggested changelog line: \bm{x} and \boldsymbol{x} are drawn bold italic, as in TeX, and serialized as \bm{x} rather than the upright \mathbf{x}.

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