Add PR testing across Swift platforms and enable all soundness checks

[jakepetroules]

Type of change

• Bug fix
• xcprojformatter (CLI) change
• Documentation update
• Test addition
• Other (describe below)

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Summary

Add PR testing across as many Swift platforms as possible, and enable all soundness checks. Modeled on swift-toolchain-sqlite's PR workflow.

Related issue

N/A

Changes

• tests job (swift_package_test.yml@0.0.15):
  • Linux and Windows with the default OS/arch matrices; Swift 5.9, 5.10, and 6.0 excluded since Package.swift requires tools version 6.1
  • macOS (Xcode and Swiftly-managed snapshot toolchains), iOS build, FreeBSD
  • Static Linux SDK, Wasm SDK, and Android SDK builds, plus Android emulator tests
• soundness job: all checks enabled except the format check (disabled for now) and the macOS docs check, which fails on the macOS runners' bash 3.2 until Fix check-docs.sh failing on macOS when an option has no values swiftlang/github-workflows#320 lands. docs_check_macos_xcode_version is set to swift_6.4 (the default 26.0 doesn't exist on the runners) so it can be re-enabled by flipping one line.
• TestPerformance.swift: guard Test.cancel behind #if compiler(>=6.3) so the tests build with Swift 6.1 and 6.2

Testing

• Existing tests pass (swift test, locally with Swift 6.2)
• New tests added for new functionality
• Tested manually: [describe steps]

Checklist

• I have read CONTRIBUTING.md
• My change is in scope (see CONTRIBUTING.md)
• Code follows the project's style guidelines
• I have reviewed my own code and can explain all changes in detail
• Documentation updated where applicable