Nexosim legacy engine

The legacy Days simulator is built on Nexosim, vendored under crates/nexosim. The current exact executor does not run Nexosim models; it uses the LP and fixed-event machinery described in /docs/internals.

Nexosim provides the legacy engine with:

• an actor-like Model API
• mailboxes for delivering events and queries to models
• typed Output<T> ports
• single-threaded and multithreaded executors
• a scheduler and monotonic simulation clock

Start with /docs/architecture/simulation-engine for the legacy topology-level view.

Code map

• Crate root: crates/nexosim/src/lib.rs
• Simulation lifecycle and stepping: crates/nexosim/src/simulation.rs
• Bench construction: crates/nexosim/src/simulation/sim_init.rs
• Ports and message delivery: crates/nexosim/src/ports.rs
• Single-threaded and multithreaded executors: crates/nexosim/src/executor.rs
• Time types: crates/nexosim/src/time.rs
• Legacy Days assembly and execution: legacy/src/topos/topo.rs

The vendored crate is version 1.0.0. Its SimInit includes three Days compatibility shims: set_hot_worker_count, set_max_groups_per_step_task, and set_time_quantum_ns.