//! Layer 3 integration test entry point (D-030, ticket #200).
//!
//! ## Three-layer test architecture (D-030 sub-decision 3)
//!
//! ```text
//! Layer 1 — Fixture-based serialization (FAST, run on every edit)
//!   Scope:    Pure unit tests. No ECS world. No subprocess.
//!   Tools:    Rust #[test] + data structures directly.
//!   Speed:    <1ms each.
//!   Files:    tests/serialization.rs, tests/information_boundaries.rs (Layer 1 tests),
//!             #[cfg(test)] mod tests within src/ modules
//!
//! Layer 2 — Mock subprocess / minimal ECS world (MEDIUM, run on every PR)
//!   Scope:    Minimal bevy App or World. Real systems, no real subprocess.
//!             IPC roundtrip over Unix socket without spawning the binary.
//!   Tools:    bevy_ecs World + Schedule, or LocalBridge with in-process simulation.
//!   Speed:    1ms–100ms each.
//!   Files:    tests/bridge_ipc.rs, tests/bridge_tcp.rs,
//!             tests/information_boundaries.rs (Layer 2 tests),
//!             tests/determinism.rs, tests/movement.rs, tests/smoke.rs
//!
//! Layer 3 — Real subprocess integration (SLOW, run daily / pre-merge)
//!   Scope:    Full binary spawned as a child process. No mocks. Real IPC.
//!             Exercises the complete path: spawn → handshake → tick → snapshot.
//!   Tools:    std::process::Command, TcpStream.
//!   Speed:    1s–15s each (process startup dominates).
//!   Files:    tests/layer3.rs, tests/integration/ (this module)
//! ```
//!
//! ## Layer 3 test guidelines
//!
//! - Always set a deadline for server startup (`LISTEN_TIMEOUT`).
//! - Always kill the child process in teardown (even on test failure — use a
//!   RAII guard or drop the handle at end of test).
//! - Use `--port 0` to get a kernel-assigned port; parse `LISTENING:{port}` from
//!   stdout to obtain the actual port.
//! - Serialize `PlayerInput` via `rmp_serde`, frame with `bridge::framing::write_framed`.
//! - Deserialize `ObserverSnapshot` via `rmp_serde` after `bridge::framing::read_framed`.
//!
//! Spec reference: D-030 (testability architecture), D-020 (subprocess IPC protocol)

// ---------------------------------------------------------------------------
// Stub: Layer 3 startup smoke test
// ---------------------------------------------------------------------------

/// Placeholder for future Layer 3 tests that require full subprocess setup.
///
/// Non-blocking tests that exercise the simulation binary end-to-end live in
/// `tests/layer3.rs`. This module is the organisational entry point for tests
/// that exercise multi-message Layer 3 scenarios (multi-tick sequences,
/// save/load roundtrip over IPC, protocol version negotiation).
///
/// See `tests/layer3.rs::server_subprocess_sends_snapshot_on_connect` for the
/// canonical Layer 3 pattern.
#[test]
fn layer3_module_entry_point_placeholder() {
    // This test exists to verify the integration module compiles and is
    // discovered by cargo test. Real Layer 3 scenario tests replace this.
    // D-030 Layer 3 stubs are acceptable until the IPC handshake (#555) lands.
}