refactor: rename project-specific preset to custom

[gabriensymons]

Summary

• rename the built-in project-specific preset identifier to custom
• update Playground values, examples, unit tests, and browser tests
• align historical changelog terminology with custom configuration language
• bump the package to 0.2.0 because this is a breaking pre-1.0 preset rename

Verification

• zero case-insensitive old-term matches in current repository files, filenames, rebuilt dist, or packed package
• 60 tests pass
• npm pack dry-run contains 25 expected files
• preset behavior is preserved under custom
• independent pre-commit review passed