[shepilov]

Fixes #5131
Fixes #5134

Problem

• Unzip worker: a file entry whose name ends with non-UTF-8 bytes ("0/\xdf") is turned into a directory and its content silently dropped #5131: the unzip worker deleted the invalid UTF-8 bytes of an entry name (utils.CleanUTF8), then used that cleaned name to decide whether the entry is a directory. photos/\xe1 (ß in CP437, as written by archivers that do not set the UTF-8 flag, common on Windows) became photos/, so it was skipped as a directory and its content silently dropped while the job reported success. Distinct names could also collide (\x82.txt and \x8a.txt both became .txt), which failed the whole job.
• Unzip worker: an archive entry that fails its CRC or data descriptor check is still saved as a file, then the job fails half-way #5134: archive/zip reports a CRC or data descriptor mismatch on the last Read, after every byte has been written. file.Close() then committed a file with unverified content, and the job failed half-way.

Solution

• The directory check reads the original header (f.Mode().IsDir() before touching the name), and f.Name is no longer mutated.
• Names that are not valid UTF-8 are decoded from CP437, the usual DOS code page; every byte maps to a rune, so names stay distinct. Valid UTF-8 names are unchanged, including those from archivers that write UTF-8 without setting the flag (macOS). golang.org/x/text is already a direct dependency. CP437 is a heuristic: names written in another code page (Latin-1, CP866, Shift-JIS...) are readable only partly, but they are no longer lost.
• When io.Copy fails after Close() committed the file, the file is destroyed (fs.DestroyFile) and the error is returned.

Test plan

• unzip keeps the files with a non UTF-8 name: photos/\xe1, \x82.txt and \x8a.txt are extracted as photos/ß, é.txt and è.txt with their content, and café.txt (valid UTF-8, no UTF-8 flag) stays café.txt
• unzip does not keep an entry with a bad checksum: the job fails with zip.ErrChecksum and a.txt does not exist
• go test ./worker/archive/ passes, including the existing path traversal tests