slog-configurator v1.2.0 — your env var names, plus logring

Init(Options) lets a caller name the environment variables this package reads.
LOG_LEVEL / LOG_FORMAT / LOG_ADD_SOURCE stay the defaults, and only the names
you give are consulted, so a stray variable cannot override yours. Options
moves the fallbacks too. The zero Options is the previous behaviour exactly, so
the blank import is unaffected.

The names used to live in struct tags -- fixed at compile time -- so changing
them meant copying the package.

logring is a bounded in-memory ring of recent records, an slog.Handler that
stacks onto the fan-out so a process can search its own logs in-process.
Bounded by bytes rather than record count, drops oversized records rather than
letting them eat the ring, INFO and above by default.

AddHandler now reports whether it stacked onto this package's fan-out. Existing
call sites compile unchanged -- discarding a result is legal Go, and there is a
compile test pinning the shapes the dependent repos use.

gonfiguration is no longer a dependency; the three settings are read from the
environment directly.