docs: record registry_env as a stopgap bundle in decision 5

[jcschaff]

Adds a paragraph to strategy decision 5, Environments for a composite of N components, saying what today's registry_env image is and where it fits.

registry_env is a single all-in-one environment: every library in the pinned list (COPASI, Tellurium, ReaDDy, multiscale actin) plus the toolkit, in one ~1.3 GB image that every composite runs in. That is the shape decision 5 rejects at scale. It is kept for now because those four coexist and cover what people submit today. The note frames it as the first pre-built bundle (rule 2), arrived at before the per-composite resolver (rule 1) exists, and sets three rules:

• It grows only while additions are conflict-free. The first library that will not co-install is the signal to build the resolver, not to force the library in.
• It is not the registry. Being in the bundle confers no curation level.
• It is retired into rule 1, not replaced beside it. Once the resolver exists, this image becomes one cached resolution among many, identified by digest like the rest.

Documentation only.

🤖 Generated with Claude Code