Run: isolate toolchain requirements between services in hybrid projects

[1lck]

Problem

In a hybrid project such as Spring Boot + Vue, a missing frontend runtime can block an unrelated backend run configuration. On Windows, selecting the Spring Boot service may report that Node is not configured even though the backend launch plan only requires Java and Maven.

The run configuration editor also lacks a complete path to discover, select, and persist generic runtimes such as Node.

Expected behavior

• Toolchain diagnostics are scoped to the run configurations that consume the requirement.
• Missing Node blocks Vue/npm services only; Spring Boot remains runnable when Java and Maven are available.
• Windows discovers generic runtimes and lets users select and persist the executable in the machine-local toolchain document.
• macOS and Windows apply the same per-configuration blocking semantics.
• Hybrid Spring Boot + Vue behavior is covered by shared, Windows, and macOS tests.

Acceptance criteria

• Backend execution does not require Node when its launch plan does not consume the Node toolchain.
• Frontend execution remains blocked with an actionable diagnostic when Node is missing or incompatible.
• Generic runtime selections are stored in .lithe/toolchains/local.json without overwriting unrelated entries.
• Running all services starts eligible services and reports failures only for affected services.
• Cross-platform contracts and fixtures document the behavior.

[1lck]

Implemented and merged into preview by #278.