Redesign Result type for cleaner Luau inference

[Word30210]

What

• Merged Ok<T> and Err<E> into a single Result<T, E> shape that carries both _value: T and _err: E on the same record (instead of two metatable-tagged union variants).
• Redefined Ok<T> as Result<T, never> and Err<E> as Result<never, E>, so the constructors return precise subtypes that upcast cleanly to Result<T, E>.
• Updated the ok / err constructors to populate the unused field with nil :: never.
• Simplified unwrap_unchecked / unwrap_err_unchecked to direct field reads — no more (self :: Ok<T>)._value style downcasts.
• Removed all :: any and :: Result.Result<T, E> workarounds from tests/std/result/.spec.luau now that subtype-based inference covers every call site.

Why

The previous design used a real metatable-tagged union Ok<T> | Err<E>. Luau's type checker does not narrow setmetatable<...> unions on a boolean discriminator field, so calling generic methods like :is_ok_and(f: (T) -> boolean) on an Err<E> (or the symmetric Err-side methods on Ok<T>) failed type checking with "no valid instantiation for T" — _value resolved to nil from the Err branch's missing field and conflicted with the closure's parameter type. As a workaround the tests had to launder these call sites through :: any, which defeated the purpose of having strict types in the first place.

Collapsing the two variants into one record gives both fields a real type at all times, with the unused side parameterized to never. Upcasting an Err<E> = Result<never, E> to Result<T, E> then works through ordinary never <: T covariance on read fields — no narrowing required from the type checker. As a side effect this also lets the unchecked methods read fields directly without a (separately broken) Result<T, E> → Ok<T> downcast.

Checklist

Required

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

Functional Validation

• Ok and Err branches still covered for every public method in tests/std/result/.spec.luau
• unwrap_unchecked / unwrap_err_unchecked exercised on the correct variant
• Runtime behavior unchanged — refactor is purely type-shape driven

Configuration & Docs

• No new dependencies; mise.toml / pesde.toml unchanged
• No sensitive values or credentials were introduced

If Applicable

• Type-surface change: Ok<T> and Err<E> are now aliases for Result<T, never> / Result<never, E> instead of independent setmetatable shapes. Downstream code that pattern-matched on the previous shape (e.g. discriminating on the absence of _value / _err) would need to switch to checking _ok. Pre-1.0, so impact is expected to be nil.