chain

type Input struct {
	ProjectDir string                 // absolute project directory
	OSEnv      []string               // os.Environ(); injected for testability
	Hostname   func() (string, error) // injected; production passes os.Hostname
	Chain      string                 // named chain to select (C4); "" or "default" = the [default]/header-less section
}

type Result struct {
	Files            []string // ordered absolute Layer-1 paths, existing only, deduped
	Vars             []string // merged "K=V" seed for the engine (OS env wins over file vars), sorted
	ComposeEnv       string   // resolved CENVKIT_ENV ("dev" default)
	ComposeEnvSource string   // where ComposeEnv came from: "shell" | ".env" | "default" (for the --overview header)
	Host             string   // resolved + sanitized host ([A-Za-z0-9._-])
}

type UnknownChainError struct {
	Name      string   // the requested chain name
	Available []string // sorted section names actually present
}