Treat leading tildes as absolute in FilePath string decoding

[shindgew]

Type of change

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

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Summary

FilePath.init(base:path:) treats a leading ~ as absolute, matching Xcode’s tilde expansion
but FilePath.init(stringRepresentation:) only looked for /

That disagreement made paths like ~/project/file.swift decode as .group and then throw Base disagrees with absoluteness of path. The same failure hit JSON decoding, round-trips of absolute tilde paths, and FilePath string literals (which use try!).

Related issue

N/A

Changes

• Infer .absolute from a leading ~ in FilePath.init(stringRepresentation:), matching init(base:path:).
• Add round-trip coverage for ~, ~/…, and ~user/….
• Keep generated relative test paths from using a leading ~, which init(base:path:) already rejects.
• 🤔 2 typos found by AI

Testing

• Existing tests pass (swift test)
• 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