Restore SingleLine group state after early exits

[OskarEichler]

Summary

Restore SingleLine's group/first-item stack when a group exits through an exception, throw, return or break, including an exception raised by the output sink. Normal output and the existing boolean return value remain unchanged.

Reproduction

q = PrettyPrint::SingleLine.new([])
q.first? # consume the parent first-item flag
begin
  q.group { raise "example" }
rescue RuntimeError
end
q.first? # currently true from the leaked child; should remain false

Verification

• rbenv Ruby 4.0.6: existing rake test, 35 tests / 54 assertions, passes before/after.
• External corpus: 40 checks, 28 failures on main before this patch, zero after; covers both parent/child flag states, four early-exit mechanisms, output failures, and normal return values.
• 2,000 generated single-/multi-line outputs remain byte-identical.
• No repository tests added or changed under the contributor's task constraint. Verification scripts are external. Other Ruby engines/versions were not executed locally.

Breaking-change notes

No API removal or normal-output change. A printer reused after an interrupted group now sees the restored parent state. Already-emitted output is not rolled back, and a closing delimiter is not synthesized after failure.