Conditionally conform Chain2Sequence to MutableCollection

[shubhransh-gupta]

Summary

Resolves #217.

Adds conditional conformance of Chain2Sequence to MutableCollection when both underlying base sequences (Base1 and Base2) conform to MutableCollection.

Motivation

When chaining two mutable collections (e.g. [1, 2, 3] and [4, 5, 6]), users expect to be able to mutate elements in-place via indices or subscripts without converting the chained sequence into an intermediate array or losing lazy execution benefits.

Changes

1. Sources/Algorithms/Chain.swift:
   • Changed base1 and base2 in Chain2Sequence from let to var.
   • Added conditional conformance extension Chain2Sequence: MutableCollection where Base1: MutableCollection, Base2: MutableCollection.
   • Implemented subscript(i: Index) with get, set, and _modify (using yield &base1[i] / yield &base2[i] for efficient in-place mutation).
2. Guides/Chain.md:
   • Updated documentation to list MutableCollection among the conditional conformances.
3. Tests/SwiftAlgorithmsTests/ChainTests.swift:
   • Added testChainMutableCollection() verifying index mutations across both chained collections.

Verification

• swift build — builds cleanly with 0 errors.