C ABI Narrow Waist

lob exposes a C ABI with plain aggregates and an opaque builder, with a header-only C++ wrapper above it that restores type-safe overloads and scoped enums. The implementation lives once, in C++; the wrapper forwards to it.

The hourglass shape keeps the solver independent of language bindings: any language with a C FFI can call lob without writing C++ or fighting name mangling, while C++ callers get ergonomics back through the wrapper. See Builder Pattern for the builder surface.