fix: select architecture-appropriate Xcode build (fixes #456)

[wmehanna]

Summary

• Add architectures field to Xcode model to capture build architecture from xcodereleases.com/data.json
• Add currentHostArchitecture to detect host CPU via uname -m
• Add firstCompatible(withVersion:hostArchitecture:) selection that prefers Universal > matching arch > first match

Problem

xcodereleases.com/data.json contains multiple entries per version:

• Xcode (Apple Silicon) with architectures: ["arm64"]
• Xcode (Universal) with architectures: ["arm64", "x86_64"]

Previously first(withVersion:) returned the first match regardless of architecture, causing Intel Macs to download arm64-only builds (issue #456).

Solution

firstCompatible(withVersion:hostArchitecture:) uses selection priority:

1. Universal build (contains both arm64 and x86_64)
2. Architecture-specific build matching host
3. First match (fallback)

Changes

File	Change
Models.swift	Add architectures: [String]? field
XcodeList.swift	Capture architectures from JSON via local XcodeReleaseEntry type
Environment.swift	Add currentHostArchitecture via uname -m
Models+FirstWithVersion.swift	Add firstCompatible(withVersion:hostArchitecture:)
XcodeInstaller.swift	Use firstCompatible instead of first
Models+FirstWithVersionTests.swift	Add tests for architecture selection

Testing

New tests verify:

• Universal preferred on both arm64 and x86_64 hosts
• Falls back to matching architecture when no Universal
• Falls back to first match when no architecture info

Closes #456