Support Xcode 27: raise minimum iOS deployment target to 15

### Description
Xcode 27 beta no longer supports iOS deployment targets below **iOS 15** — building against an SDK whose `Package.swift` declares a lower floor produces `The iOS deployment target ... is set to X.0, but the range of supported deployment target versions is 15.0 to ...` warnings on every build.

This package currently declares **`.iOS(.v12)`** in its `platforms` array, which triggers these warnings under Xcode 27.

### Request
Please raise the SwiftPM `platforms` floor to `.iOS(.v15)` (and the matching deployment target in the project/podspec) so the package builds cleanly under Xcode 27.

### Environment
- Xcode 27 beta
- SwiftPM integration