- Require PHP >= 8.2, PHPUnit 11
- Implement __serialize/__unserialize on all data structures (Pair, Vector,
  Deque, Stack, Queue, Set, Map, PriorityQueue) for PHP 9.0 readiness
- Add __clone to Map and PriorityQueue for proper deep cloning
- Suppress array_sum deprecation warnings on PHP 8.3+
- Use static return types on Sequence methods (merge, filter, map, reversed,
  slice, sorted) so Vector::merge() correctly returns Vector, etc.
- Add return types to __toString(): string, __debugInfo(): array
- Fix Capacity::shouldDecreaseCapacity docblock

Thanks to @simPod, @hopeseekr-contribs, and @Bilge for their contributions.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Replace v1 types (Vector, Deque, Stack, Queue, PriorityQueue) with Seq
and Heap. Replace Hashable with Key interface. Make Pair readonly. Add
functional constructors (seq, map, set, heap). Simplify Map and Set
internals. Remove Collection and Sequence interfaces.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Closes #113

Fix function redeclare fatal error