Honor recursive local transfers for filesystem paths

[OskarEichler]

Summary

Apply recursive filesystem copying before the stream/file distinction for local transfers.

Reproduction and verification

upload!(Pathname(directory), destination, recursive: true) currently tries to stream a directory and raises EISDIR. download!(directory, destination, recursive: true) also ignores recursive and attempts a stream copy. Candidate uses FileUtils.cp_r for the requested recursive operation, including the default download destination.

• Focused branch against master 28e04dc73de911374a23c4fc3fd5ef34012dcd63; installed release is 1.25.1 (e35241b7a3acdf3df4d70442b4a2e69bbcfff155).
• Relevant existing tests pass on this isolated branch. Combined release candidate passes 218 unit tests / 3,405 assertions and nine local functional tests / 13 assertions, plus 12 focused checks, 8,507 model assertions and 22 native local path checks. Combined totals cover this and the other focused changes, not this PR alone.
• No test/spec files added or changed; focused checks are external scratch fixtures. Docker/SSH functional tests were not run. A wrapper disables Docker startup and after-run teardown during the unit/local suites.
• Original RuboCop 0.52.1 fails with Ruby 4/Psych argument compatibility. With an external compatibility shim, changed files pass lint; full lint retains an unrelated existing Lint/BlockAlignment cop error in nil_cache.rb. No full green-lint claim.
• Package build preserves 1.25.1, Ruby >=2.5, dependency metadata and all 96 package paths. Ruby 2.5 syntax parsed by upstream lint tooling; no older-runtime or remote-platform execution claim.

Breaking changes / compatibility / limitations

Intentional correction: recursive local downloads now honor the option, and recursive Pathname uploads work. Non-recursive IO uploads/downloads retain stream behavior and input offsets. Recursive transfer requires filesystem paths, not an IO destination. No remote transfer implementation changes.

No production access, real SSH sessions, deployment or concurrency increase.