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.toml

@@ -29,6 +29,9 @@ members = [
     "crates/runtime/arje-bus",
     "crates/runtime/arje-cas",
     "crates/runtime/arje-wasm",
+    "crates/runtime/arje-brain-rules",
+    "crates/runtime/arje-brain-cognitive",
+    "crates/runtime/arje-brain-audit",
     "crates/runtime/arje-brain",
     "crates/runtime/arje-echo",