feat(comparator): Support custom Comparator, defaults to bytewise

[alexsnaps]

Custom comparators are fully implemented and tested.

New file: src/comparator.rs — Comparator trait (compare, name, find_shortest_separator, find_short_successor) + BytewiseComparator default.

Threaded through ~40 sites across 14 files:

• Options::comparator: Arc<dyn Comparator>
• cmp_internal_keys — accepts &dyn Comparator
• SkipList / Memtable — stores comparator, uses it for entry ordering and key lookups
• Block / BlockIter — comparator for binary search in seek
• Table — comparator for user-key equality in get
• TableBuilder — find_shortest_separator for index block separators, find_short_successor for last entry
• MergingIterator / DbIterator — comparator for merge ordering and direction switching
• VersionSet — file sorting; VersionEdit encodes/decodes comparator name; recover rejects mismatches
• TableCache — stores comparator for lazy Table::open
• All lib.rs free functions — compaction, overlap detection, flush placement

3 new tests (272 total): reverse-bytewise in-memory, reverse-bytewise persistent with reopen, comparator mismatch rejection.