feat(server): initialize Rust/bevy_ecs simulation server

Server boilerplate epic (276) complete. Establishes the Rust simulation
server foundation per D-020 (subprocess/IPC architecture).

Structure:
- bevy_ecs 0.18 + bevy_app 0.18, MessagePack serialization (rmp-serde)
- SimulationPlugin with deterministic resources: SimulationTime (D-031),
  SimRng (D-030), InputQueue (D-010)
- Core IPC types: ObserverSnapshot, PlayerInput, SimBridge trait (D-020)
- CauseChain production component for provenance tracking (D-030)
- SimulationTier types with LRU eviction support (D-026)
- NPC 10-axis model components (D-024)
- 15 tests: inline unit tests + integration smoke/serialization tests
- make ci-server passes (clippy, fmt, build, test)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/bridge/mod.rs

@@ -0,0 +1,40 @@
+// Bridge module - Client-server communication
+// Implements D-020 subprocess/IPC architecture
+// MessagePack serialization for Rust<->Godot communication
+
+use bevy_app::prelude::*;
+
+pub mod types;
+pub use types::*;
+
+/// Error type for bridge operations
+#[derive(Debug, thiserror::Error)]
+pub enum BridgeError {
+    #[error("serialization error: {0}")]
+    Serialization(#[from] rmp_serde::encode::Error),
+    #[error("deserialization error: {0}")]
+    Deserialization(#[from] rmp_serde::decode::Error),
+    #[error("transport error: {0}")]
+    Transport(String),
+}
+
+/// Abstracts transport layer (D-020)
+/// Implemented by LocalBridge (stdio) and future NetworkBridge
+pub trait SimBridge: Send + Sync {
+    /// Send an observer snapshot to the client
+    fn send_snapshot(&self, snapshot: &ObserverSnapshot) -> Result<(), BridgeError>;
+
+    /// Receive player inputs from the client
+    fn receive_inputs(&self) -> Result<Vec<PlayerInput>, BridgeError>;
+}
+
+/// Bridge plugin for client-server communication
+/// Abstracts transport layer (LocalBridge/NetworkBridge)
+pub struct BridgePlugin;
+
+impl Plugin for BridgePlugin {
+    fn build(&self, _app: &mut App) {
+        // Stub implementation - will be populated in phase 2
+        tracing::debug!("BridgePlugin initialized");
+    }
+}

server/src/bridge/types.rs

@@ -0,0 +1,56 @@
+// Bridge type definitions
+// ObserverSnapshot: data crossing the client-server boundary
+// PlayerInput: semantic actions from client
+
+use serde::{Deserialize, Serialize};
+
+/// The ONLY data structure crossing the client-server boundary (D-020)
+/// Contains all information visible to the observer at a given tick
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ObserverSnapshot {
+    /// Simulation tick when this snapshot was produced
+    pub tick: u64,
+    /// All entities visible to the observer
+    pub entities: Vec<VisibleEntity>,
+}
+
+/// A visible entity in the simulation
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct VisibleEntity {
+    pub entity_id: u64,
+    pub x: f32,
+    pub y: f32,
+    pub z: i32,
+    pub kind: EntityKind,
+}
+
+/// Category of visible entity
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum EntityKind {
+    Npc,
+    Object,
+    Terrain,
+}
+
+/// Semantic player actions, not raw key events (D-020)
+/// Timestamped for deterministic processing
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PlayerInput {
+    /// Tick when this input was issued
+    pub tick: u64,
+    /// The action to perform
+    pub action: PlayerAction,
+}
+
+/// Player action variants
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub enum PlayerAction {
+    MoveNorth,
+    MoveSouth,
+    MoveEast,
+    MoveWest,
+    Interact,
+    UsePerceptionMode(String),
+    Pause,
+    Unpause,
+}