slogging v1.7.0 — the handler API says what it does

MultiWriterHandler was never a multi-writer: two writers, routed by level.
handlers.Handler is that thing named honestly, with several writers per stream
now actually available via Stdout()/Stderr() options, and the split point a
parameter rather than a hardcoded branch.

AddSink and SetOutput are now separate calls. Adding a second console handler
used to print every line twice, and replacing it used to delete your sinks.

Fixes a runtime log level that never took effect: the level is a slog.Leveler
resolved per check, so a *slog.LevelVar bumped later works as documented.

Breaking, deliberately shipped in v1 -- nothing outside this repo imports it,
so a /v2 path would cost a second import sweep to protect nobody.