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/cause_chain.rs

@@ -0,0 +1,72 @@
+// Cause chain tracking system
+// Production component for information provenance (D-030)
+// Tracks monologue triggers, journal entries, debug causality
+
+use bevy_ecs::prelude::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Component, Debug, Clone, Serialize, Deserialize)]
+pub struct CauseChain {
+    pub causes: Vec<Cause>,
+}
+
+impl CauseChain {
+    pub fn new() -> Self {
+        Self { causes: Vec::new() }
+    }
+    pub fn with_cause(mut self, cause: Cause) -> Self {
+        self.causes.push(cause);
+        self
+    }
+    pub fn push(&mut self, cause: Cause) {
+        self.causes.push(cause);
+    }
+    pub fn latest(&self) -> Option<&Cause> {
+        self.causes.last()
+    }
+}
+
+impl Default for CauseChain {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Cause {
+    pub tick: u64,
+    pub kind: CauseKind,
+    pub description: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+pub enum CauseKind {
+    Observed,
+    Heard,
+    Informed,
+    Inferred,
+    Background,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn chain_preserves_order() {
+        let chain = CauseChain::new()
+            .with_cause(Cause {
+                tick: 1,
+                kind: CauseKind::Observed,
+                description: "Saw NPC".into(),
+            })
+            .with_cause(Cause {
+                tick: 5,
+                kind: CauseKind::Inferred,
+                description: "NPC still inside".into(),
+            });
+        assert_eq!(chain.causes.len(), 2);
+        assert_eq!(chain.latest().unwrap().tick, 5);
+        assert_eq!(chain.latest().unwrap().kind, CauseKind::Inferred);
+    }
+}