Add macOS platform support to resolve dependency conflicts

[didiergarcia]

Summary

Adds macOS 10.15+ platform support to resolve dependency conflicts when SegmentAmplitude and analytics-swift are used together in multi-platform projects.

Problem

When both Analytics Swift and the Segment Amplitude plugin are added to the same project as dependencies, the project fails to build due to a platform version mismatch:

The package product 'Segment' requires minimum platform version 10.15 for the macOS platform, 
but this target supports 10.13 (in target 'SegmentAmplitude' from project 'SegmentAmplitude')

This occurs because:

• analytics-swift requires macOS 10.15+
• SegmentAmplitude did not declare macOS support in Package.swift
• Swift Package Manager defaults to an incompatible baseline version

Solution

Package.swift:

• Added .macOS(.v10_15) to platforms array to match analytics-swift dependency requirement

AmplitudeSession.swift:

• Made UIKit import conditional (#if !os(macOS))
• Refactored lifecycle handling into platform-specific extensions:
  • iOS/tvOS: iOSLifecycle protocol with UIApplication? parameters
  • watchOS: watchOSLifecycle protocol with WKExtension parameters
  • macOS: macOSLifecycle protocol with no parameters (new!)

All platforms use the same session management logic (startNewSessionIfNecessary(), timeout handling, etc.) while respecting platform-specific lifecycle APIs.

Test Plan

• Build succeeds for macOS target (swift build)
• Build succeeds for iOS Simulator
• Created test project with both analytics-swift and SegmentAmplitude dependencies - builds successfully
• Runtime initialization works correctly on macOS
• Verified no regression - existing iOS/tvOS/watchOS functionality preserved
• Test in CI pipeline (GitHub Actions)
• Manual testing of session tracking on macOS app (if available)

Changes

• Package.swift: Added macOS 10.15+ to platforms array
• Sources/SegmentAmplitude/AmplitudeSession.swift: Platform-specific lifecycle extensions

Files Changed: 2 files, +37 insertions, -17 deletions

Backward Compatibility

✅ Fully backward compatible - no breaking changes to existing iOS, tvOS, or watchOS functionality.

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