Allow 'then error' without a reason

[woksin]

Changed

• then error no longer requires a message. A bare then error asserts that the scenario is rejected without naming why, which is the shape most rejection scenarios have — the reason lives in the specification's own name. then error "<message>" is unchanged for the case where a specific message is asserted. (Allow 'then error' without a reason for a rejection the specification does not name #46)
• SpecificationErrorSyntax.Name is nullable, so an unnamed reason stays distinguishable from an empty one. (Allow 'then error' without a reason for a rejection the specification does not name #46)

[woksin]

Reviewer notes — left open for a call on the design, not merged, since it is a language decision.

Answering the two questions raised in #46:

May a bare then error and a then error "…" appear in the same specification? Yes, as implemented. They are independent assertions in a list that was already zero-or-more, and there is no reading under which mixing them is ambiguous — one says "rejected, reason unspecified here", the other "rejected with this message". Forbidding it would need a rule to justify it, and I could not find one.

Does the printer round-trip a bare then error unchanged? Yes, and there is a spec asserting it. SpecificationErrorSyntax.Name became string? specifically so the printer can tell the two apart — null prints as then error, "" still prints as then error "". Collapsing them onto one representation would have made generated documents un-diffable, which is the requirement the issue calls out.

Scope. The regex change is ^then\s+error(?:\s+"…")?$, so every existing document parses exactly as before. The only breaking surface is SpecificationErrorSyntax.Name becoming nullable — a consumer that dereferences it now needs a null check. Labelled minor on the basis that the nullability annotation change does not break compilation for consumers; say the word if you would rather it were major.

Verification — dotnet build Debug and Release both 0 errors / 0 warnings (Release treats warnings as errors); dotnet test 585 passing, 0 failing. Debug carries one pre-existing IDE0305 warning in for_ScreenplayWriter/when_inspecting_assembly_visibility.cs that predates this branch and is untouched here.