Three test layers against live webinstall.dev APIs:
1. Unfiltered release API — OS/ext vocabulary, version format
2. Filtered release API — correct package for specific OS/arch
3. Installer script rendering — WEBI_* vars, download URLs
47 pass, 2 known format-preference diffs (node .pkg/.7z vs .tar.gz/.zip),
5 known Go-cache improvements (excluding .deb/.pem non-installable formats).
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