Treat local path objects as paths while retaining IO behavior

[OskarEichler]

Summary

Normalize local path-protocol objects before selecting the upload/download IO path. Keep real File/IO objects as streams, including their current positions and caller ownership.

Reproduction and evidence

Closes #29. With a Pathname source, a six-byte upload in three-byte chunks repeatedly rereads the first chunk and never completes; a bounded 30-transition simulation reproduces it. A Pathname destination receiving abc, then def, retains only def because Pathname#write reopens the file. Both public-entrypoint simulations pass after this patch. Combined release integration also transfers binary files both ways through local /usr/bin/scp processes.

Compatibility / breaking changes

No version/dependency change. Objects implementing to_path but not to_io are treated as paths. File/IO objects with to_io retain streaming behavior. Custom objects that deliberately implemented both to_path and read/write without to_io will now be treated as paths.

Verification and limits

• This focused branch passes the existing upstream suite: 36 tests / 70 assertions, Ruby 4.0.6, net-ssh 7.3.3, test-unit 3.7.8 and Mocha 2.1.0; all Ruby commands used rbenv.
• The combined release-based consumer candidate passes the suite with frozen string literals enabled, 10 focused cases, 2,906 model assertions, 24 URI-component assertions and 19 local scp/URI assertions. Combined evidence includes other separately proposed fixes and existing upstream frozen-string corrections; it is not attributed to this patch alone.
• No repository tests added/modified under the consuming project's explicit no-new-tests policy. Reproductions/models ran externally. No deployed servers, SSH connections or production data were used.
• All seven runtime files parse with the Ruby 2.6 parser; actual older-Ruby/Windows/remote-server matrices were not run locally. No upstream lint config exists; targeted lint has the same eight pre-existing warnings, with no new warnings.
• Existing upstream PRs/issues and contribution guidance were checked. Maintainer CI results are pending; no CI pass is claimed.