Narrow `Coroutine['data']['exit']` resolver from `Result<unknown>` to `Result<void>`

What

In lib/types.ts:

export interface Coroutine<T = unknown> {
  data: {
    exit(resolve: Resolve<Result<unknown>>): void;
    ...
  };
}

The exit callback's resolver should be Resolve<Result<void>>, not Resolve<Result<unknown>>.

Why

The exit signal carries either "done OK" or "an error happened" — never a value. Every source of data.exit already produces Result<void>:

• noopExit resolves with Ok() (which is Result<void> per the overload in lib/result.ts)
• effect.enter() is declared to return (resolve: Resolve<Result<void>>) => void (types.ts:361)

The wider Result<unknown> type forces explicit casts at definition sites (e.g., custom Effect definitions whose exit returns Ok()), even though the value channel is unused.

Impact

The only read site is lib/coroutine.ts:51, which inspects exitResult.ok and assigns the Err variant to data.resumeWith (which is Result<unknown>). Result<void>'s Err variant is structurally identical to Result<unknown>'s Err variant, so narrowing the type is non-breaking.

Out of scope

Not blocking — the workaround is a cast at the call site.

Start in lib/types.ts at the Coroutine data.exit resolver and compare its type with the Result sources described in the issue, including noopExit and effect.enter() at types.ts:361. Check lib/coroutine.ts:51 to confirm the read site still handles the Err variant; done means the resolver uses Resolve<Result> without requiring the noted casts.