`apple/swift-syntax` is now only a redirect to `swiftlang/swift-syntax`.
Declaring the old URL while a co-resolved dependency (mlx-swift-lm) uses
the new one makes SwiftPM see two URLs for one package identity — a
warning today, a hard error in newer toolchains. Switch to the canonical
`swiftlang` URL so the whole graph agrees. Same repo, same version range.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>