Runtime and process-management helpers for Codex artifact generation.

This crate has two main responsibilities:

• locating, validating, and optionally downloading the pinned artifact runtime
• spawning the artifact build or render command against that runtime

• src/client.rs Runs build and render commands once a runtime has been resolved.
• src/runtime/manager.rs Defines the release locator and the package-manager-backed runtime installer.
• src/runtime/installed.rs Loads an extracted runtime from disk and validates its manifest and entrypoints.
• src/runtime/js_runtime.rs Chooses the JavaScript executable to use for artifact execution.
• src/runtime/manifest.rs Manifest types for release metadata and extracted runtimes.
• src/runtime/error.rs Public runtime-loading and installation errors.
• src/tests.rs Crate-level tests that exercise the public API and integration seams.

• ArtifactRuntimeManager Resolves or installs a runtime package into ~/.codex/packages/artifacts/....
• load_cached_runtime Reads a previously installed runtime from a caller-provided cache root without attempting a download.
• is_js_runtime_available Checks whether artifact execution is possible with either a cached runtime or a host JS runtime.
• ArtifactsClient Executes artifact build or render requests using either a managed or preinstalled runtime.