feat(math): implement m:phant phantom converter

[Abdeltoto]

Closes #2608

Summary

• Implements the m:phant OMML-to-MathML converter (invisible spacing placeholder)
• Full phantom maps to <mphantom>; partial dimension control maps to <mpadded> with zeroed width/height/depth
• Supports m:show, m:zeroWid, m:zeroAsc, and m:zeroDesc property flags
• Registers the converter in MATH_OBJECT_REGISTRY
• Adds 3 unit tests covering full phantom, visible with zeroed width, and invisible with zeroed height

Spec reference

ECMA-376 Section 22.1.2.81

Test plan

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

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[caio-pizzol]

@Abdeltoto thanks for contributing this! the phantom converter is a nice addition and the code is clean — matches Word perfectly on 7 out of 10 test cases.

one thing to fix: when <m:show> is missing, content renders invisible but should be visible. this affects the most common phantom pattern (zeroing a dimension without explicitly setting show). left an inline comment with a one-line fix, comparison screenshots, and 10 Word-native test files.

[caio-pizzol]

when <m:show> is missing from the phantom properties, the content should be visible — but right now it renders invisible. this is because both sides of the OR are false when show is undefined.

this breaks the most common phantom use case: zeroing descent without hiding content (the example in the spec). I tested 10 Word-native .docx files covering every property combination — these three don't match Word:

file	what happens	should happen
phantom-01 (no properties)	content hidden	content shown
phantom-03 (zeroed descent)	content hidden inside radical	content shown inside radical
phantom-09 (fraction + zeroed descent)	content hidden	content shown

phantom-01
SuperDoc:

Word:

phantom-03
SuperDoc:

Word:

Suggested change

	const isVisible = isOnOffTrue(show?.attributes?.['m:val']) || (show != null && !show.attributes);
	const isVisible = show == null || isOnOffTrue(show?.attributes?.['m:val']) || (show != null && !show.attributes);

tests 1 and 4 will need updating too — they expect the current (wrong) behavior.

test files attached below if you want to verify locally.
phantom-test-files.zip