Build a shared wheel once in the test suite

[kurtmckee]

This cuts the setup times for tox environments roughly in half (from ~2s to ~1s when running locally), and reduces local tox p invocations from ~18s to ~14s.

---

Python 3.14 on Ubuntu examples in CI:

BEFORE [citation]

  py314-crypto: OK (17.23=setup[11.03]+cmd[6.20] seconds)
  py314-nocrypto: OK (6.45=setup[5.68]+cmd[0.77] seconds)
  congratulations :) (23.72 seconds)

AFTER [citation]

  py314-crypto: OK (13.84=setup[7.74]+cmd[6.10] seconds)
  py314-nocrypto: OK (4.31=setup[3.61]+cmd[0.70] seconds)
  congratulations :) (18.21 seconds)

---

Usage summaries in CI:

BEFORE [citation]

Runtime: 25m 14s

AFTER [citation]

Runtime: 22m 35s