Fix block LaTeX rendering with SwiftMath display mode

[manemajef]

Summary

SwiftMathBridge currently renders both inline $ … $ and block $$ … $$
formulas using SwiftMath's text mode.

That gives block formulas inline typography: limits sit beside large operators,
fractions remain small, and operators do not scale as expected.

What changed

The parser already distinguishes inline and block LaTeX. This PR carries that
information through LatexRenderer:

• inline and table formulas use .inline
• block formulas use .display
• SwiftMathBridge maps the mode to SwiftMath's labelMode
• the render mode is part of the cache key, preventing inline and display
  results for the same formula from colliding

The Markdown source remains unchanged; no \displaystyle injection or content
transformation is involved.

Compatibility

Existing LatexRenderer conformers remain source-compatible. The new
mode-aware overload defaults to the original renderer method, so renderers that
do not distinguish the two modes continue working unchanged.

Direct calls to render(latex:fontSize:theme:) retain inline behavior.

Validation

• swift build
• swift test --parallel — 473 core tests and the SwiftMath regression test pass

[manemajef]

I rebased this onto the current main and tightened the patch after another
compatibility review. The original LatexRenderer method remains required, and
the new mode-aware overload has a one-way fallback to it, so existing conformers
continue to compile and behave as before.

I also removed unrelated documentation churn, reduced the SwiftMath regression
tests, added the required changelog entry, and reran swift build plus the full
test suite successfully. @luca-chen198, when you have a chance, could you take
another look?