Options

oneenv uses the functional-options pattern — the zero config is already sensible, options are applied in order, and a later option wins over an earlier one.

Option	Description
`WithFiles(names...)`	`.env` files to read (default `.env`); later files override earlier keys. A missing default `.env` is not an error.
`WithEnvFiles()`	Enable the environment-aware cascade: also read `<base>.local`, `<base>.<env>`, `<base>.<env>.local` (all optional).
`WithEnvVar(names...)`	Which env variables name the active environment for `WithEnvFiles` (default `APP_ENV`, then `GO_ENV`).
`WithPrefix(p)`	Restrict lookups to keys carrying a prefix, e.g. `APP_` maps `env:"PORT"` to `APP_PORT`.
`WithOverride()`	Let `.env` values overwrite variables already in the process env (default: existing wins).
`WithExpand()`	Enable `${VAR}` / `$VAR` expansion inside values. Secret and `,noexpand` fields are never expanded.
`WithExpandStrict()`	As `WithExpand()`, but a reference to an undefined variable fails with `ErrUnknownVariable` instead of expanding to `""`.
`WithRequired()`	Treat every field as required, as if each carried `,required`.
`WithTagKey(k)`	Change the struct tag key (default `env`).
`WithLookuper(l)`	Swap the env source — pass a `MapLookuper` for hermetic tests.
`WithTypeParser[T](fn)`	Register a custom parser for a specific type `T`.
`WithMutator(m)`	Transform each raw value before decoding (receives a `context.Context`).
`WithValidator(fn)`	Run a validation callback on the decoded struct — plug in any validator, zero-dep.
`WithContext(ctx)`	Context passed to mutators (also via `LoadContext` / `ParseContext`).