Bump Swift tools and SwiftSyntax to 6.3

[bradleymackey]

Summary

• Updates the Swift package tools version to 6.3.
• Moves the SwiftSyntax dependency requirement to the 603 release line.
• Refreshes both checked-in SwiftPM lockfiles to resolve SwiftSyntax at 603.0.2.

Impact

This keeps the package manifest and macro dependency aligned with the Swift 6.3 toolchain while keeping the package and workspace lockfiles in sync.

Validation

• swift package update swift-syntax
• swift package resolve
• make format
• make lint