RTK Rust Design Patterns

Patterns that apply to RTK's filter module architecture. Focused on CLI tool patterns, not web/service patterns.

Pattern 1: Newtype (Type Safety)

Use when: wrapping primitive types to prevent misuse (command names, paths, token counts).

// Without Newtype — easy to mix up
fn track(input_tokens: usize, output_tokens: usize) { ... }
track(output_tokens, input_tokens);  // Silent bug!

// With Newtype — compile error on swap
pub struct InputTokens(pub usize);
pub struct OutputTokens(pub usize);
fn track(input: InputTokens, output: OutputTokens) { ... }
track(OutputTokens(100), InputTokens(400));  // Compile error ✅