Files — Changelog, Download URL, and Save-CurseForgeFile

[MariusStorhaug]

Parent: #8

Implement changelog retrieval, download URL resolution, and the convenience file download function.

Acceptance criteria:

• Get-CurseForgeFileChangelog returns file changelog in HTML format
• Get-CurseForgeFileDownloadUrl returns the download URL string
• Save-CurseForgeFile resolves the download URL and downloads the file to a specified path
• -Path parameter on Save defaults to current directory with original filename
• -PassThru switch returns the downloaded file info
• Unit tests cover all three functions

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• Create src/functions/public/Files/Get-CurseForgeFileChangelog.ps1 — GET /v1/mods/{modId}/files/{fileId}/changelog
• Create src/functions/public/Files/Get-CurseForgeFileDownloadUrl.ps1 — GET /v1/mods/{modId}/files/{fileId}/download-url
• Create src/functions/public/Files/Save-CurseForgeFile.ps1 — composite (resolves URL then Invoke-WebRequest -OutFile)
• Create tests/Files/Save-CurseForgeFile.Tests.ps1