Refactor temporary execution paths

[timzhong1024]

Summary

• unify temporary project launch paths across Node, Python, Rust, and .NET
• collapse duplicate process result and error representations
• remove forwarding-only backend constructors and test-only display helpers
• remove the serde_json runtime dependency used only for a static manifest
• move CommonJS validation to the CLI boundary

Impact

• 132 additions and 360 deletions relative to main
• no user capability removed
• Node now displays its explicit vp env exec launch command
• invalid --commonjs combinations fail before source input is read

Validation

• cargo fmt --all -- --check
• cargo test --locked (30 passed)
• cargo clippy --locked --all-targets -- -D warnings
• cargo package --locked --allow-dirty
• node --check bin/run-code.js
• npm pack --dry-run
• real dependency smoke tests for Node, Python, Rust, Go, and .NET
• cwd, env-file, quiet mode, argument forwarding, exit codes, and missing-tool hints verified