BDD DSL: Prepare named Helmfile environments

[sbaum1994]

Why

Profile and topology features repeatedly copy a fixture into a stack environment directory and then apply a visible YAML values table. The chosen stack, environment, fixture, and values are the contract; destination-path construction and repeated edit plumbing are stable mechanics.

Scope

Add a step that prepares a named Helmfile environment for a named stack from an explicit fixture and visible values table.

Acceptance criteria

• Add the step and table shape to the documented DSL catalog.
• Keep the stack name, environment name, fixture path, and every YAML key/value visible.
• Validate stack and environment names without relying on an ambient working directory.
• Snapshot the destination through the suite ledger before writing it.
• Reuse the existing YAML update and interpolation behavior.
• Do not hide profile selection, image sources, installation, or control-plane versus compute-plane workflow choices.
• Add focused helper, handler, validation, and ledger tests.
• Migrate suitable fixture-copy and YAML-update sequences.
• Pass affected wiring tests and live feature tests.

Parent: #858