docs(uuid): document that v7 UUIDs are not monotonic within a millisecond

[tomas-zijdemans]

Documents an existing limitation of @std/uuid/v7. No code changes.

generate() fills everything after the 48-bit timestamp with random bits. In RFC 9562 terms it uses none of the optional monotonicity methods from section 6.2, so two UUIDs created in the same millisecond sort in arbitrary order relative to each other. Sortability is the main reason people reach for v7, and neither the module doc nor the generate() doc mentioned the caveat. Now both do, with a link to the RFC section.

If in-millisecond ordering is ever wanted, a counter (method 1 or 2 of section 6.2) can ship as an additive option without changing the current API. This PR takes no position on whether that is worth doing.

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (ca58f94) to head (ee876fd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7286      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         617      617              
  Lines       51637    51637              
  Branches     9359     9359              
==========================================
- Hits        49075    49074       -1     
  Misses       2021     2021              
- Partials      541      542       +1     

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