Word30210/fix trait

[Word30210]

What

• Introduce trait.inherent(class_table, methods): C & M for merging a class's own methods (kept in a separate table) into the class table; methods already present are not overwritten.
• Add brief doc comments above Trait<D>, define, impl, and inherent in src/std/trait/init.luau.
• Update tests/std/trait/.check.luau to express the new convention — own methods live in a dedicated table and are merged via inherent — and demonstrate that inherent's C & M return surfaces those methods at the type level.
• Add three focused .inherent cases to tests/std/trait/.spec.luau (copies own methods / returns the class table / does not overwrite an existing method) and reshape the integration case to follow the new convention.

Why

#8 documents the limitation of the previous convention. To silence the duplicate-method ambiguity that arises when an implementer's class table and a trait's Requires<Self> both contribute the same method into the final intersection, __index had to be widened with __index = Class :: {}. That cast erased the class table's type from typeof(Prototype), so any method the class defined for itself — one not required by any trait — became invisible on instances even though it existed at runtime.

trait.inherent carves own methods out into a separate, dedicated table whose type is captured by inherent's C & M return (and remains directly available as typeof(methods_table)). With own methods isolated from the trait-required surface, implementers can compose their public instance type from independent pieces — class fields, the metatable, the own-methods table, and each Trait.For<Self> — without forcing the duplicate-method intersection that motivated the :: {} workaround in the first place. define and impl are unchanged; the addition is purely additive.

Closes #8.

Checklist

Required

• lute run ./scripts/checker.luau -- ./src ./tests ./scripts passes
• lute run ./scripts/tester.luau passes
• I linked the related issue if one exists (for example: Closes #19)

Functional Validation

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

Configuration & Docs

• User-facing docs were updated where applicable (README.md / README.ko.md / CLAUDE.md)
• New dependencies / configuration are reflected in pesde.toml / mise.toml
• No sensitive values or credentials were introduced

If Applicable

• Breaking type-surface or API changes are clearly described in this PR