chore: benchmark against html-to-text, inscriptis and rust html2text

[hownowstephen]

Adds three widely used non-Go converters to the comparison benchmarks, so textplain's speed and output can be read against more than the two Go libraries.

• workers_test.go runs each library as a long-lived worker fed length-prefixed frames over stdin, so timings exclude interpreter startup. Workers join implementations in TestMain, so BenchmarkConvert and TestFidelity pick them up unchanged.
• testdata/{node,python,rust} holds the workers with pinned versions: html-to-text 10.0.1, inscriptis 2.7.4, html2text 0.17.1.
• Wrapping libraries wrap at 65, like textplain.

Reviewer notes:

• A worker that isn't set up is skipped with a message, so the benchmarks still run without Node, Python or Rust. Setup commands are in benchmarks/README.md.
• Timings include a pipe round trip of about 10–30µs per document, measured with an echo worker. The new libraries take 180µs–1ms per document on the corpus emails, so the round trip is a small share.
• The benchmarks module isn't run by CI, so nothing changes there.