feat: added a Drift trait and a "default" ConstantDrift implementation

graph.d2

@@ -0,0 +1,64 @@
+vars: {
+  d2-config: {
+    layout-engine: elk
+    # Terminal theme code
+    theme-id: 300
+  }
+}
+
+History: {
+  shape: class
+
+  agents: "HashMap<Index, Agent>"
+  batches: "Vec<Batch>"
+}
+
+Batch: {
+  shape: class
+
+  skills: "HashMap<Index, Skill>"
+  events: "Vec<Event>"
+  time: "i64"
+  p_draw: "f64"
+}
+
+Event: {
+  shape: class
+
+  teams: "Vec<Team>"
+  weights: "Vec<Vec<f64>>"
+  evidence: "f64"
+}
+
+Team: {
+  shape: class
+
+  items: "Vec<Item>"
+  output: "f64"
+}
+
+Item: {
+  shape: class
+
+  agent: "Index"
+  likelihood: "Gaussian"
+}
+
+Skill: {
+  shape: class
+
+  forward: "Gaussian"
+  backward: "Gaussian"
+  likelihood: "Gaussian"
+  elapsed: "i64"
+  online: "Gaussian"
+}
+
+History -> Batch
+
+Batch -> Skill
+Batch -> Event
+
+Event -> Team
+
+Team -> Item