Why v7 UUIDs beat v4 for database keys (and how to hand-roll both)

I build one small browser tool a day and write down what I learned. Day 25 was a UUID generator. What started as "make some random IDs" turned into a proper look at how the bits are laid out, and why the newer v7 format is quietly the better default for a primary key.

Live tool:

Takeaway: a UUID is 16 bytes with a couple of fixed bits, and picking v7 over v4 for a key is a free performance win.