- collapse() now applies env vars for keys present in fileConfig, not
just defaultConfig. Previously, env vars couldn't override file values
unless a default was also set for that key.
- SetDefault no longer pollutes overrideConfig; it correctly resolves
the value by checking override > env > file > default.
- Remove unused explicitDefaults field and UseExplicitDefaults method.
- Add IsSet, AllKeys, AllSettings methods + package-level wrappers.
- Add missing package-level wrappers: Get, SetBool, SetString,
SetConfigDir, WithEnvPrefix.
- Add tests for all new methods and the env-over-file-without-default
fix.
The documented precedence is Set > env > file > defaults, but collapse()
was using maps.Copy(ccm, mapConfig) which let file values (and Set values,
stored in the same map) unconditionally overwrite env values.
Split mapConfig into fileConfig (from ReadInConfig) and overrideConfig
(from Set/SetString/SetBool). collapse() now applies layers in correct
order: defaults, then file, then env (for known keys), then overrides.
Added TestPrecedenceChain to verify the full layering.
