refactor(interpreter): resolve strict Clippy findings

[shayyz-code]

Goal

Remove all strict Clippy diagnostics from the legacy interpreter without changing language behavior.

Scope

• Replace the direct ToString implementation with behavior-compatible Display.
• Add Default implementations that delegate to existing constructors.
• Apply behavior-preserving option, collection, borrow, closure, and conditional simplifications in src/interpreter.rs.
• Add a focused legacy value-formatting specification.
• Split and update the Clippy cleanup roadmap status.

Acceptance criteria

• Strict Clippy reports no diagnostics from src/interpreter.rs.
• Formatting, compilation, and the complete test suite pass.
• No lint suppressions or language behavior changes.