Always-mint session cookie, rename _bs_verified -> _bs_session #37

Job	Run time
pytest (not browser) + short soak	8m 36s
pytest (browser) — headless Chromium	2m 14s
docs/ is up to date with sources	8s
8h soak (manual)	0s
fuzz (cookie + robots, 30m each, manual)	0s
	10m 58s

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