Add EqEdit triangle symbol support

[kilomanyo]

Summary

• map the documented uppercase TRIANGLE token and source-evidenced lowercase triangle token to LaTeX \triangle
• preserve operator/symbol semantics in EqEdit preview MathML without rewriting serialized XML
• cover reader, renderer, authoring round-trip, and ordinary-identifier negative cases

Verification

• full upstream gate: 3135 passed, 18 skipped, 1 xfailed
• Ruff, mypy, pyright, contract/hygiene checks, build, twine check, and clean-wheel smoke passed

The change is a general EqEdit token-map correction; it has no document-, school-, hash-, or full-script-specific behavior.

[airmang]

Thanks for this PR — the diagnosis is exactly right (latex2mathml does classify \triangle as an identifier while \angle already comes out as an operator, so the minimal scope here is well chosen), and I really appreciate the negative test cases around identifier splitting. Nice work.

While playing with the branch locally I ran into one edge case I'd like to fix before merging:

\text{...} content gets rewritten too. The regex substitution runs over the whole LaTeX string, so a quoted EqEdit literal containing \triangle breaks:

render_equation(r'"\triangle" + x')
# main: <mtext>\triangle</mtext><mo>+</mo><mi>x</mi>
# PR:   <mtext>\mathop{\triangle</mtext><mi>}</mi><mo>+</mo>...

Rare input, but the tokenizer accepts it and main renders it cleanly. Skipping \text{...} arguments before the substitution (plus a negative test for this script) should cover it. Happy to handle this in a follow-up if you'd prefer.

One question on the authoring direction: since _invert is first-seen, \triangle now authors as lowercase triangle. We got burned by this once with forall — the real Hancom build renders the lowercase form as literal text, which is why authoring emits FORALL (ff79bd8). Do you know whether lowercase triangle has been render-verified on a real build? If it's only been observed in the reading direction, it might be safer to add an explicit "\triangle": "TRIANGLE" override matching the documented spelling until we can verify.

Everything else looks good to me — once the \text case is covered I'm happy to merge.