feat: add rayon feature

[ParkMyCar]

Closes #432.

Adds an optional rayon feature so CompactString works with rayon's parallel iterators the same way String does. It implements both FromParallelIterator and ParallelExtend for every item type rayon supports for String — char, &char, &str, String, Box<str>, Cow<str> — plus the Item = CompactString variants the issue asked for, including collecting/extending a String from CompactStrings (allowed by the orphan rule since CompactString is local).

Each impl delegates the actual parallel collection to String (reusing rayon's optimized string collectors) and then converts, so the behavior matches String exactly — the two proptests assert that directly (collect/par_extend into CompactString vs String produce identical results). The feature pulls in std (rayon needs it), and there's no unsafe in the module.

Tests are #[cfg_attr(miri, ignore)] — rayon's thread pool is far too slow under Miri, and there's no unsafe here for it to check anyway.

Verified: --features rayon builds, the 7 tests pass (5 unit + 2 String-parity proptests at 5k cases), --all-features suite green (331 + 84), clippy clean under -D warnings, fmt clean, and --no-default-features --features rayon compiles (the feature enables std). Added README and CHANGELOG entries.