docs and networking: improve viewport reach detection

[ZitouniNidhal]

Summary

This PR improves both contributor setup and viewport address classification.

Changes

• Document Homebrew installation for XcodeGen and SwiftLint.
• Clarify that project.yml is the source of truth and show when to run xcodegen generate.
• Correctly classify IPv4-mapped IPv6 hosts (::ffff:w.x.y.z) using the existing IPv4 private-range and loopback rules.
• Add regression coverage for mapped LAN, loopback, and public addresses.

Verification

• Added focused XCTest coverage in ViewportReachTests.
• The full Xcode test suite could not be run here because this session has no local workspace or macOS/Xcode environment.
• Existing mpx CLI repository link preserved.

[jhen0409]

Thanks!