perf: pre-allocate Partition result slices

Adds capacity hints to both keep and reject slices in Partition
function to prevent repeated allocations during append operations.

Changes:
- keep: make([]T, 0) → make([]T, 0, len(values))
- reject: make([]T, 0) → make([]T, 0, len(values))

Impact:
- Reduces allocations from O(log n) to O(1) for each slice
- Improves performance by eliminating slice growth overhead
- Minimal memory overhead as worst case is original slice size

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

partition.go

@@ -3,8 +3,8 @@ package underscore
 // Partition splits the slice into two slices: one whose elements all satisfy predicate
 // and one whose elements all do not satisfy predicate.
 func Partition[T any](values []T, predicate func(T) bool) ([]T, []T) {
-	keep := make([]T, 0)
-	reject := make([]T, 0)
+	keep := make([]T, 0, len(values))
+	reject := make([]T, 0, len(values))
 
 	for _, v := range values {
 		if predicate(v) {