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>

crates/runtime/arje-brain-audit/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "arje-brain-audit"
+version = "0.0.1"
+edition.workspace = true
+license.workspace = true
+publish.workspace = true
+description = "Capa de accountability del brain: audit log con hash chain anclado al CAS."
+
+[dependencies]
+arje-brain-rules = { path = "../arje-brain-rules" }
+arje-brain-cognitive = { path = "../arje-brain-cognitive" }
+arje-cas = { path = "../arje-cas" }
+serde = { workspace = true }
+serde_json = { workspace = true }
+ulid = { workspace = true }
+tokio = { workspace = true }
+anyhow = { workspace = true }