Use frozenset for OMML direct tag lookup

[Amitanand983]

Summary

Replaces __direct_tags from tuple to frozenset in the DOCX OMML math converter (oMath2Latex) to improve membership lookup efficiency in a hot code path.

Motivation

process_unknow() checks if stag in self.__direct_tags while recursively traversing OMML nodes.
For math-heavy documents, this check executes frequently.

• tuple membership is linear scan: O(n)
• frozenset membership is hash-based on average: O(1)

frozenset is immutable like tuple, and better represents an unordered constant set used only for membership checks.

Change

• packages/markitdown/src/markitdown/converter_utils/docx/math/omml.py
  • __direct_tags changed from tuple to frozenset
• No API or behavior change

Validation

• pre-commit run --all-files: passed (black)
• hatch test: 331 passed, 4 skipped, 1 pre-existing unrelated failure (test_speech_transcription)

Risk

• Single-file, single-concept change
• No change to converter output semantics