Build a real public API and installable package surface

[audexdev]

Summary

The repository has include/lac/lac.hpp and include/lac/version.hpp, but they are currently empty. Consumers must include headers from src/, which makes internal implementation files the de facto public API.

Affected areas

• include/lac/lac.hpp
• include/lac/version.hpp
• CMakeLists.txt
• README usage examples

Work items

• Define the intended public API surface.
• Export stable encoder/decoder entry points from include/lac/lac.hpp.
• Add version constants or macros to include/lac/version.hpp.
• Avoid requiring consumers to include src/ headers directly.
• Add CMake install/export rules and package config if the library is meant to be consumed externally.

Acceptance criteria

• A small external example can include only public headers and link against lac.
• Public headers document ownership, error behavior, supported formats, and experimental status.
• CMake exposes a coherent installable target or clearly documents that install support is not yet available.