feat(userspace-rt): add runtime crate with entry point, panic handler, and allocator re-export

Snake and hello apps now use extern crate userspace_rt — no more boilerplate assembly or panic handlers in app code.

crates/userspace-rt/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "userspace-rt"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+types = { workspace = true }
+userspace = { workspace = true }