symbulator 0.6.18

No change to any answer: nothing under symbulator/ moved but one comment
and the version number. This release exists so that PyPI's page carries the
new classifier and README.

• Out of beta (#471). Symbulator 9 left the beta it had been in since its
  public launch on 28 Aug 2026; the classifier is now
  Development Status :: 5 - Production/Stable, and Python 3.13 and 3.14 are
  listed beside 3.9–3.12.
• The repository, ready for review (#470). A GitHub Actions workflow runs
  the suite on Python 3.9 to 3.14, with the ahkab ground-truth tests in a job
  of their own; CONTRIBUTING.md, CODE_OF_CONDUCT.md and CITATION.cff
  added; the JOSS paper and its fact-check in paper/.
• README. The Tests section names the command that counts the suite
  instead of a count that had gone stale ("48 tests across six files"; there
  are 26 files), and says why the ahkab and IPython tests skip in a plain
  install. Two sentences that still said a side of the circuit behind a port
  is "reported floating" now say it is given a local reference, as it has
  been since 0.5.32. The quick start's transient uses 1'u, so the printed
  answer matches its comment exactly. Install names the Python versions and
  the extras.

On PyPI: https://pypi.org/project/symbulator/0.6.18/ (wheel sha256 f93423548d1d...).