feat(yachay): notebooks reproducibles — yachay-core + demo

yachay-core: notebook como secuencia de celdas (orden de lectura) +
DAG de dependencias (orden de ejecución). Celdas markdown/código/embed
con content_hash BLAKE3; editar una propaga staleness a descendientes;
digest Merkle por celda (content_hash ‖ digests upstream) y
notebook_digest que certifica reproducibilidad. Demo CLI en apps/yachay.

14 tests. Sin kernel ni UI, #![forbid(unsafe_code)].

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

Cargo.toml

@@ -143,6 +143,11 @@ members = [
     "crates/modules/matilda/matilda-config",
     "crates/modules/matilda/matilda-plan",
 
+    # ============================================================
+    # modules/yachay/ — Notebooks computacionales reproducibles
+    # ============================================================
+    "crates/modules/yachay/yachay-core",
+
     # ============================================================
     # modules/nakui/ — ERP matemático (categórico)
     # ============================================================
@@ -256,6 +261,7 @@ members = [
     "crates/apps/agorapura",
     "crates/apps/badu",
     "crates/apps/matilda",
+    "crates/apps/yachay",
 ]
 
 [workspace.package]