feat(shuma): shuma-sysmon — muestreo de CPU/memoria con historial

Lee /proc/stat y /proc/meminfo; calcula uso de CPU (delta entre
muestras) y de memoria; mantiene un History circular para la curva
del monitor. Parseo puro (parse_cpu_stat/parse_meminfo) separado de
la lectura de archivos → testeable sin tocar el sistema.

8 tests. #![forbid(unsafe_code)], cero deps de UI.

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

Cargo.toml

@@ -181,6 +181,7 @@ members = [
     "crates/modules/shuma/shuma-core",
     "crates/modules/shuma/shuma-intent",
     "crates/modules/shuma/shuma-line",
+    "crates/modules/shuma/shuma-sysmon",
     "crates/modules/shuma/shuma-shell-render",
 
     # ============================================================