Initialize Cargo workspace and scaffold sim-core/proto/server/cli/ui crates

[lordhippo]

Motivation

• Create a root Rust workspace and scaffold the requested crate layout under crates/ to provide a minimal, composable simulation stack.
• Expose a very small public surface for early integration: sim-core::Simulator, sim-server::SimulationService, and sim-cli entrypoints for realtime and fixed-step running.
• Add an optional visual frontend crate (sim-ui) that is feature-gated and wired as an optional dependency for the CLI.

Description

• Added a workspace Cargo.toml with members for crates/sim-core, crates/sim-proto, crates/sim-server, crates/sim-cli, and crates/sim-ui, and wired workspace-local dependencies.
• Implemented crates/sim-core with a minimal WorldState and Simulator that supports step and world access.
• Implemented crates/sim-proto with SimulationRequest, SimulationResponse, and ProtocolError types for simple protocol validation.
• Implemented crates/sim-server with SimulationService that validates SimulationRequest and delegates stepping to Simulator.
• Implemented crates/sim-cli library entrypoints realtime_entrypoint and fixed_step_entrypoint and a small binary dispatcher supporting realtime and fixed-step modes; added sim-ui as an optional, feature-gated integration and scaffolded a placeholder crates/sim-ui crate.

Testing

• Ran cargo check, which initially failed due to a network index fetch error but after switching to workspace-local dependency wiring cargo check completed successfully.
• Ran cargo test, which completed successfully with all test runs reporting ok (no tests were defined yet).

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Codex Task