You're means you are. Your shows that something belongs to you. ACT English tests this as a word-choice item, not as a style preference. The diction pattern is in word choice on ACT English.
Read it out loud as “you are”
You're late works because “you are late” works. Your late does not. Bring your calculator fails the same test: “bring you are calculator” is nonsense, so your is the possessive.
The apostrophe replaces the a in are. It does not create a possessive. Noun possessives are a separate rule, covered in apostrophes.
The neighbor that looks the same
Who's and whose split the same way: contraction versus possession. That pair is in who's versus whose. If the choice is about tone rather than a swapped spelling, use formality and word choice.
The bottom line
Substitute “you are.” If the sentence still works, you need the apostrophe.
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