Support RO-Crate 1.3 metadata

[sapunyangkut]

Summary

• add final RO-Crate 1.3 to the supported version selector
• retain RO-Crate 1.2 as the library default
• extend regression coverage to regular and detached 1.3 crate writes and reads

Fixes #258.

Validation

• preserved task-owned evidence for this exact upstream base and patch: targeted write/version coverage 10 passed; full suite 208 passed, 3 skipped; flake8 clean
• current worktree: git diff --check and python -m compileall -q rocrate/model/metadata.py test/test_write.py passed

The current system environment does not have the existing arcp dependency or flake8, so pytest and flake8 were not rerun here; no dependency was installed.

AI assistance disclosure

This patch, tests, commit message, and pull request text were developed and checked with OpenAI Codex assistance. No independent human self-review is claimed; maintainer review is requested before merge.