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.lock

@@ -14875,6 +14875,21 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7a5a4b21e1a62b67a2970e6831bc091d7b87e119e7f9791aef9702e3bef04448"
 
+[[package]]
+name = "yachay"
+version = "0.1.0"
+dependencies = [
+ "yachay-core",
+]
+
+[[package]]
+name = "yachay-core"
+version = "0.1.0"
+dependencies = [
+ "blake3",
+ "serde",
+]
+
 [[package]]
 name = "yamux"
 version = "0.12.1"