Gating v2 on the global basin alone would have degraded every run to v1 on a
deployment that provisions a basin per organization and sets no global one,
even though S2 is fully working there. `resolveStreamBasin` already resolves
run, session and organization basins ahead of the global setting, so either
source now satisfies the basin requirement.
Splits the pure resolver out from the env lookup so the version matrix can be
tested without reaching for `env.server`.
Matt Aitken
dab295751b