iOS is declared in Package.swift but FormatoCore does not build for iOS #27

Found while working on mac4DSTEM/SpectroKit#19 (2026-10-28).

Package.swift declares .iOS(.v18), but xcodebuild -scheme <package> -destination generic/platform=iOS build (run from SpectroKit, which depends on FormatoKit 0.7.0) fails:

Sources/FormatoCore/GuardedXML.swift:77:51: error: cannot find type 'XMLDocument' in scope

XMLDocument is macOS-only Foundation. Either drop the iOS platform line, or gate the XML path with #if os(macOS) and build iOS in CI. SpectroKit is dropping its own iOS claim (#17), since it cannot build for iOS until this is fixed.