refactor: decompose compile module

[zharinov]

Summary

• Extract Compiler struct and core impl to compiler.rs
• Extract CompileError and CompileResult to error.rs
• Extract tests to compile_tests.rs
• Reduce mod.rs to thin facade with re-exports

Why

The compile/mod.rs was 427 lines mixing core types, implementation, and tests. This decomposition follows the project's facade pattern where mod.rs contains only module declarations and re-exports.