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/hello/Cargo.toml

@@ -9,11 +9,4 @@ test = false
 bench = false
 
 [dependencies]
-userspace = { workspace = true }
-
-[profile.release]
-panic = "abort"
-opt-level = "s"
-
-[profile.dev]
-panic = "abort"
+userspace-rt = { workspace = true }