Solo stdlib: date_today() -> ISO YYYY-MM-DD

[hyperpolymath]

Sub-issue of #45. time() gives only a Unix float; tooling needs a calendar date string for stamping generated files. Add date_today() -> String (YYYY-MM-DD). Repro in #45.

[hyperpolymath]

Implemented and merged to main (user signed off 2026-05-18). See #45 for the consolidated summary.