Load StringIO before rendering parse trees

[OskarEichler]

Summary

Require the standard StringIO library used by Tree#to_s. A fresh Ruby process that only requires raabro currently raises NameError while printing a tree; loading unrelated libraries first can hide the missing require.

Reproduction

ruby -Ilib -rraabro -e 't = Raabro.str(:value, Raabro::Input.new("x"), "x"); puts t.to_s'

Before: uninitialized constant Raabro::Tree::StringIO. After: prints 1 :value 0,1 "x".

Verification

• Reviewed master baseline, each individual patch, and the combined installed-release branch: existing suite passes 94 tests / 169 assertions, zero failures.
• Ruby 4.0.6 via rbenv, Probatio 1.6.2. No new or modified test files.
• Focused temporary repros, Ruby syntax and gem packaging checks pass. Targeted Lint reports the same 13 pre-existing warnings on baseline and fixed source, with no new warnings.
• Consumer branch is based on installed 1.5.0 rather than importing unrelated master changes.

No production operations. Other Ruby/platform combinations were not run locally; no full supported-platform claim. Current open/closed issue/PR searches found no overlapping fix.

Breaking changes

None intended. Adds the missing explicit require for a standard/default library; public methods and tree formatting remain unchanged.