Serialize FTP typecodes without mutating the URI

[OskarEichler]

Summary

Insert the FTP typecode into the returned string before its query/fragment suffix. Avoid temporarily replacing @path while serializing.

Reproduction

URI('ftp://example.test/file;type=i').freeze.to_s raises FrozenError before this change and returns the original URI string afterward. The prior serializer wrote @path and restored it only after calling super.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 93 tests / 3,039 assertions / zero failures or errors, both baseline and this isolated patch.
• 12 checks cover repeated frozen serialization and complete instance-state preservation, with absent/i/a/d typecodes, an escaped absolute path and query/fragment suffixes.
• Syntax and git diff --check pass. Supplemental Lint adds no findings against the 31-finding baseline.
• No test/spec files added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions ran externally.

Compatibility and limitations

No wire-format change for ordinary serialization. Frozen URI instances now serialize successfully. No concurrent benchmark or throughput claim is made. No public API removal, dependency change or Ruby minimum change.

Based on master 696df43b0be4c05d3a0dcf7db582126c915d860d. Other Ruby/OS runtimes were not executed locally. No production traffic or live mail/FTP services were used. Existing related PRs/issues were checked; this is a focused contribution, not exhaustive behavioral coverage.