CLI: live JSON events for convert --json and watch --json

[yavuzmeteafsar]

With --json, filee convert prints nothing until the whole job is done, so a front end (or a script) can't show progress or tell which file finished. filee watch accepts --json but ignores it, and since it never ends, one JSON document isn't possible there anyway.

Proposal, following the contract of filee engines --json:

convert --json: stdout stays one document at the end, unchanged. stderr gets one JSON object per line while the job runs:

{"event":"progress","source":"a.heic","progress":0.42}
{"event":"file","source":"a.heic","state":"done","outputs":["a.jpg"]}
{"event":"file","source":"b.heic","state":"failed","error":"..."}

watch --json: events go to stdout, one JSON object per line: started (folder, target, output folder), file (same fields as above) and stopped on Ctrl+C.

The data is already there (FileResult.Progress and ConversionQueue.JobUpdated), so this would stay inside Filee.Cli, with filee help convert / filee help watch and tests updated. Existing --json output doesn't change.

If this sounds good, I'd be happy to open the PR.

[KnifeLemon]

Sounds good, Yavuz, please open the PR. The shape fits what engines --json already does: one document on stdout, one JSON object per line on stderr.

A few details so it lines up with what's there:

• Same names as the existing output. engines --json progress lines call it fraction (0–1, rounded to 4 places), so please use fraction rather than progress.
• file events = the objects in files[]. Same fields and values as the final document: source, state (done / skipped / failed / cancelled), outputs, and error / errorKey / notice / missingFonts / needsEngines when they apply. A front end can then reuse one parser, and nothing in the final document changes.
• Not too chatty. The engine installer reports about every 150 ms; something similar for progress (and only when the value changes) keeps stderr readable.
• watch --json: events on stdout as you describe (started, file, stopped). Please make sure nothing else is printed to stdout in that mode, so every line parses.
• Help and tests: filee help convert / filee help watch, and a test that reads the lines back as JSON.