Environment validation surface: plugins declare the environment variables they need, and the kernel validates them at startup, failing fast on violations.
validate(spec: Readonly<Record<string, EnvVarSpec>>): void
Declares and validates environment variables.
Call this from IPlugin.register. The kernel validates every
declared spec once, after all plugins have registered and before the init
hooks run, so a spec declared later (e.g. from an onInit hook) is never
checked.