chore: split Persons using deterministic uuid

[bretthoerner]

Problem

No problem, per say, but I discussed with @tiina303 and one benefit of using deterministic Person UUIDs is that we can split distinct_ids back out to their original Person UUID, and drop an override for them, and things should just work.

Changes

Use the deterministic UUID for Person rows in split_person

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Existing

[posthog-bot]

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[bretthoerner]

@tiina303 This is ready, I checked and all other PRs are failing with this (totally unrelated) Django test failure.