chore: bump MSRV to 1.81.0 (due to dependencies)

Cargo.toml

@@ -5,7 +5,7 @@ members = ["dagx-macros"]
 name = "dagx"
 version = "0.1.0"
 edition = "2021"
-rust-version = "1.78.0"
+rust-version = "1.81"
 authors = ["Stephen Waits <steve@waits.net>"]
 license = "MIT"
 description = "A minimal, type-safe, runtime-agnostic async DAG executor with compile-time dependency validation"