Refactor HashTableSeparateChaining: encapsulate Entry, fix iterator, simplify API (#1316)

Make Entry a private static inner class. Remove redundant method aliases
(add/insert/hasKey) in favor of put/containsKey. Add dedicated modCount
for fail-fast iteration instead of using size as a proxy. Rewrite iterator
so hasNext() is idempotent. Use single-pass iterator removal in removeEntry
to avoid double linear scan. Remove unnecessary bucket.clear() in resize.
Add educational comments on key implementation details.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: williamfiset