ObjectId ↔ UUID converter

Convert an ObjectId to a UUID by zero-padding, reverse it exactly, and decode what the 12 bytes actually contain.

The conversion zero-pads the 12 ObjectId bytes to 16, so it is reversible and preserves sort order by creation time. Note that the result is not a version-4 UUID — the version and variant bits carry ObjectId data. A NoSQLSync migration does not do this: it stores an ObjectId as its 24-character hex string. Use this when you have decided you want UUID keys and will convert every reference yourself.

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