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/apps/snake/Cargo.toml

@@ -11,3 +11,4 @@ bench = false
 [dependencies]
 types = { workspace = true }
 userspace = { workspace = true }
+userspace-rt = { workspace = true }