Return main to the Rayon version (pool reuse) for 0.2.0; own threads stay on develop #20

Maintainer decision (2026-10-30):

• Keep publishing the Rayon version for now.
• Think through what Rayon cannot do before deciding on own threads.
• The own-threads version (Own worker threads: a process-wide reserve of std::thread workers instead of a private Rayon pool (#15) #16) is kept on the develop branch (302c37d). If it ships later, it ships as 0.3.0.

Work

• Return main to the Rayon version of [experiment, do not merge] Reuse the private Rayon pool between pools of one width (faer#107 comparison) #19 (9dfcbdb: Rayon pool reuse per width, libgomp's waits) through an ordinary PR, without a force-push. This covers src/, tests/ and docs/.
• docs/design/own-threads.md leaves main and stays on develop.
• Version 0.2.0. It is a minor bump because of a contract change: dropping a default pool keeps its threads in a cache instead of joining them.
• README "Upgrading from 0.1" is rewritten for the Rayon version.
• After the merge: publish 0.2.0 and tag v0.2.0 (approved by the maintainer), then close [experiment, do not merge] Reuse the private Rayon pool between pools of one width (faer#107 comparison) #19 as superseded.

Refs #16, #17, #18, #19.