Add large-year parity tests and README updates

[abhishekpradhan]

Summary

• add GNU-parity tests for large-year date parsing and relative behavior
• add timezone-rule tests for extended year handling
• update README examples and return-type docs to describe ParsedDateTime

Notes

• stacked on top of PR Add extended runtime + ParsedDateTime API for large years #1 in this fork

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test -q

[abhishekpradhan]

Closing this fork-stacked PR as superseded by the clean upstream follow-up flow.\n\nCurrent state:\n- Runtime/API is tracked in https://github.com/uutils/parse_datetime/pull/263\n- The tests/docs follow-up is prepared on branch codex/large-year-parse-datetime-pr3-upstream and will be opened upstream after PR2 merges.