Mark mutating partition functions as @discardableResult

[shubhransh-gupta]

Description

Resolves #67.

Adds @discardableResult to all mutating partitioning methods on MutableCollection (stablePartition(count:subrange:by:), stablePartition(subrange:by:), stablePartition(by:), and partition(subrange:by:)), matching the Swift Standard Library's MutableCollection.partition(by:) (SE-0120) and Rotate.swift in Swift Algorithms.

Detailed Design

When calling in-place partition algorithms, callers often perform the mutation for its reordering effect and do not always require the returned pivot Index. Without @discardableResult, ignoring the return value emits an unnecessary compiler warning (Result of call to 'stablePartition' is unused).

Adding @discardableResult removes this friction without impacting callers that do capture the pivot index.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary