feat(math): implement m:eqArr equation-array converter

[Abdeltoto]

Closes #2607

Summary

• Implements the m:eqArr OMML-to-MathML converter for equation arrays (systems of equations)
• Maps m:eqArr to a left-aligned <mtable columnalign="left"> with one <mtd> cell per row
• Unlike m:m (matrix), equation arrays use m:e elements directly as rows
• Returns null for empty arrays
• Registers the converter in MATH_OBJECT_REGISTRY
• Adds 2 unit tests covering standard system of equations and empty array

Spec reference

ECMA-376 Section 22.1.2.34

Test plan

• vitest run passes for omml-to-mathml.test.ts
• Linter and formatter pass (lefthook pre-commit)