feat(env): add pluggable world environments

feat(env): add pluggable world environments

Add an EnvironmentProvider contract so the workspace can open a
pluggable 3D world by name, with discovery through a registry that
pairs built-in providers with ServiceLoader classpath plug-ins.

The --env=<name> flag resolves a provider, reports unknown or
unavailable environments, and lets the workspace own the provider
lifecycle. A built-in grid provider validates the contract without
external data. Test-driver flags are recognized anywhere on the
command line so they can coexist with --env, and start.sh rebuilds the
modules before launching the Fallout 4 environment.