Switching target language looked like the extension breaking. It was a silent
95-second wait.

Chrome downloads a language pack per pair, on first use. Having fetched
English to Traditional Chinese buys nothing for English to Japanese: that is a
fresh download, and every one of the seven non-English targets this extension
offers reports `downloadable` on a clean profile. Meanwhile the badge sat at
"Translating 0/2" and the selection panel said "Translating…", so the only
reasonable conclusion was that it had stopped working.

The plumbing existed and was never connected: translateBuiltin has taken an
onDownloadProgress callback since 2.7.0 and the broker never passed one. It
does now, over a new `downloading` message on the stream port. The whole-page
badge reads "Downloading language pack 42%"; the selection panel says the same
and adds that it happens once per language.

Measured on a clean profile with the target set to Japanese: progress appeared
at 7 s, the page finished at 95 s, and the translation was correct. The wait
is Chrome's and cannot be removed. Being told about it is the whole fix.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>