[WIP] Clarify flush logic in code comments

[Copilot]

• Fix translator.ts: guard offsetHeight layout flush with element existence check instead of removing it entirely
• Fix m_position.ts: add early return when $what.length is 0 to prevent crash
• Create translator.test.ts: tests for resetPosition (no-throw with empty/missing element, flush preserved for real elements)
• Create m_position.test.ts: tests for positionUtils.setup including setup(what, options) with non-existent element

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