Implement trait utility

[Word30210]

What

• Add src/std/trait/init.luau - a trait utility for defining and implementing Rust-style traits:
  • define(defaults) - declares a trait from its table of default-method implementations
  • impl(methods, trait) - checks an implementer's method table against the trait's required-method contract, then copies the default methods in
• Add tests/std/trait/.spec.luau - runtime tests for define / impl
• Add tests/std/trait/.check.luau - type-level tests for the required-method contract

Why

rusty-luau ports Rust idioms, and traits are a foundational one. This PR lands just the core define / impl primitives so later traits can build on them. Conventions documentation and worked examples are intentionally left out for now - they'll be revisited once more traits (Display, Iter<T>, ...) are implemented.

Checklist

Required

• lute run ./scripts/checker.luau -- ./src ./tests ./scripts passes
• lute run ./scripts/tester.luau passes

Closes #3.

Functional Validation

• Tests cover the changed behavior (.spec.luau and .check.luau both added)
• Both happy and edge paths verified where applicable

Configuration & Docs

• User-facing docs were updated where applicable - N/A (no README / CLAUDE.md change; trait conventions doc deferred to a later PR)
• New dependencies / configuration are reflected in pesde.toml / mise.toml - N/A (none added)
• No sensitive values or credentials were introduced

If Applicable

• Breaking type-surface or API changes are clearly described - N/A (new module, no existing API touched)