fix(clipboard): restore osc52 fallback

[mathiasbourgoin]

Closes #144.

Summary

• make copy_both report whether any native clipboard path succeeded
• restore OSC 52 fallback when native clipboard tools are unavailable
• add a PATH-isolated fallback regression test

Tests

• dune fmt
• dune build
• dune exec test/test_clipboard.exe

Notes

• No CHANGELOG entry in this branch to keep the six independent bugfix PRs conflict-free; a single rollup changelog entry can be added after merge.