feat: add Rust secure-exec runtime library

[NathanFlurry]

Summary

• Adds crates/secure-exec-rs, a Rust implementation of the secure-exec runtime library with V8-based JavaScript execution
• Extracts shared IPC protocol into crates/secure-exec-ipc for reuse between the existing V8 runtime and the new Rust library
• Implements core abstractions: FileSystem, Permissions, NetworkAdapter, CommandExecutor, SystemDriver, and RuntimeDriver traits
• Includes integration tests covering exec, module exports, error handling, and filesystem bridge round-trips

Test plan

• cargo test in crates/secure-exec-rs passes all integration tests
• cargo build succeeds for both secure-exec-rs and secure-exec-ipc
• Existing v8-runtime crate still builds and passes tests after IPC extraction