Configuration loading and validation for [sdivi-rust](https://github.com/geoffgodwin/sdivi-rust).
Provides `Config::load_or_default` (5-level precedence chain), `BoundarySpec` YAML reader,
and per-category threshold overrides with mandatory `expires` fields.
Part of the `sdivi-rust` workspace. See the [workspace README](../../README.md) for full documentation.