0304 — Logging goes through a caller-supplied interface with an io.Writer-backed default

Context. Before this, the library had no operator-facing log surface; adding one meant choosing between hardcoding a logging implementation and letting callers supply their own.

Decision. pkg logging accepts a logger interface, not a hardcoded implementation. A default io.Writer-backed implementation ships for callers with no opinion.

Consequences. Callers plug in their own logging stack without the library importing any logging framework; callers who do not care get working output with zero setup.