feat(math): implement m:acc accent converter

[Abdeltoto]

Closes #2604

Summary

• Implements the m:acc OMML-to-MathML converter (diacritical marks above a base expression)
• Maps m:acc to MathML <mover accent="true"> with a stretchy <mo> for the accent character
• Defaults to circumflex (U+0302) when m:chr is absent, matching Word behavior
• Registers the converter in MATH_OBJECT_REGISTRY
• Adds 3 unit tests covering tilde, default circumflex, and dot accent

Spec reference

ECMA-376 Section 22.1.2.1

Test plan

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