BREAKING CHANGE: Logging output now goes to either file or console, not both
Logging changes:
- Rename MultiWriter to LogWriter for clarity
- Change logging to write to either file or console exclusively
- Improve log initialization message format
- Add better documentation for logging behavior
Test organization:
- Move tests from directory_processor.rs to separate test modules
- Follow modern Rust convention using tests.rs instead of mod.rs
- Create proper test directory structure under src/tests/
- Make necessary struct fields pub(crate) for testing
This commit improves code organization and makes logging behavior more
conventional by directing output to a single destination.
