Provides a virtual filesystem abstraction that overlays a base directory with an override directory, used to support provider-specific template overrides (e.g., ga vs beta). The implementation is self-contained, cleanly separates concerns, and demonstrates a reusable pattern for merging hierarchical file trees without mutation.

Implements a generic helper to zero out struct fields that match their zero/default values before custom YAML marshaling. Uses reflection to traverse struct fields and recursively omit defaults. This solves a common problem in configuration serialization and is easily extracted for any project needing clean YAML output with implicit defaults.

Defines a Runner interface for executing shell commands with support for a working directory stack and mockable test implementation. The interface is minimal and general-purpose, enabling easy substitution in tests. The concrete implementation uses os/exec safely. This pattern is directly reusable in any Go CLI tool that needs testable command execution.

Implements a rule-based engine for detecting breaking changes in Terraform resource schemas. Defines a BreakingChangeRule interface with Check methods, and registers rules for field removal, type changes, min/max alterations, and required→optional transitions. The rule registration pattern is clean and extensible; new rules can be added without modifying the engine. This design is applicable to any schema version comparison problem.