refactor: validate runtime fault network choices

[0xcadams]

What

Parse the runtime-load scenario and network impairment path into finite TypeScript choices before fault orchestration.

Why

Runtime fault scenarios already had a Zod enum, but setup used raw SCENARIO and NETWORK_PATH environment strings and passed a loose path into the fault runtime. That left typing behind at the entry point.

Changes

• Derive NetworkPath from its Zod schema and parse environment input before constructing the typed fault runtime.
• Parse the combined internal and external runtime scenarios once while retaining the existing fault event schema.
• Exercise both valid paths and invalid or missing path input without changing the impairment command sequence.