refactor(brain): A2 — split arje-brain en 3 sub-crates

DAG de dependencias limpio (modularidad horizontal):
- arje-brain-rules     — rules + engine + dispatch (motor determinista)
- arje-brain-cognitive — observer + crystallize (estadística)
- arje-brain-audit     — audit chain → CAS (accountability)
- arje-brain           — umbrella de integración (introspect +
                         autopromote + metrics + loader)

Habilitador clave: TimedEvent movido de observer.rs a rules.rs
(engine lo necesitaba, era el único acoplo que rompía el DAG).

arje-brain re-exporta la API de los 3 sub-crates: arje-zero y chasqui
(consumidores) no requieren cambios. cargo check --workspace verde.
24 tests del brain pasan (4 rules + 6 cognitive + 5 audit + 9 umbrella).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -299,6 +299,9 @@ name = "arje-brain"
 version = "0.0.1"
 dependencies = [
  "anyhow",
+ "arje-brain-audit",
+ "arje-brain-cognitive",
+ "arje-brain-rules",
  "arje-card",
  "arje-cas",
  "base64 0.22.1",
@@ -311,6 +314,43 @@ dependencies = [
  "ulid",
 ]
 
+[[package]]
+name = "arje-brain-audit"
+version = "0.0.1"
+dependencies = [
+ "anyhow",
+ "arje-brain-cognitive",
+ "arje-brain-rules",
+ "arje-cas",
+ "serde",
+ "serde_json",
+ "tokio",
+ "ulid",
+]
+
+[[package]]
+name = "arje-brain-cognitive"
+version = "0.0.1"
+dependencies = [
+ "arje-brain-rules",
+ "serde",
+ "serde_json",
+ "ulid",
+]
+
+[[package]]
+name = "arje-brain-rules"
+version = "0.0.1"
+dependencies = [
+ "anyhow",
+ "arje-card",
+ "base64 0.22.1",
+ "serde",
+ "serde_json",
+ "tracing",
+ "ulid",
+]
+
 [[package]]
 name = "arje-bus"
 version = "0.0.1"