result: add sentinel errors and errors.Is support #30

Where errors stand now #

There is one type: *Error{Result}, and a resultError helper that wraps a C result code. It's a solid foundation, but three things are missing:

1. No errors.Is support. Every failure allocates a fresh *Error, so there is nothing to compare against. There are a ton of var ge *Error; errors.As(err, &ge) && ge.Result == ResultNoValue references in the codebase and the test suite has a helper for exactly this.
2. There is no operation context. All 71 return resultError(C....) references produce the same string, "ghostty: no value", so a real app can't tell whether the grid ref, the formatter, or the snapshot decoder failed. Why don't we capture the Op on the struct (see my follow-up PR).
3. Callback errors are wrapped inconsistently. The formatter wraps the reader's original error so errors.Is(err, io.EOF) works. Other paths that can carry a callback error do not follow the same shape.

Design #

This PR adds one exported sentinel per non-success Result (ErrNoValue, ErrInvalidValue, ...) and gives *Error an Is method that matches on Result.

This means errors.Is(err, ErrNoValue) now works directly and through fmt.Errorf %w wrapping. The existing errors.As usage is unchanged.