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/src/main.rs

@@ -1,24 +1,9 @@
 #![no_std]
 #![no_main]
 
-use core::arch::global_asm;
-
-global_asm!(
-    r#"
-.section .text.entry
-.globl _start
-_start:
-    call main
-    j _start
-"#
-);
+extern crate userspace_rt;
 
 #[unsafe(no_mangle)]
-extern "C" fn main() -> ! {
-    userspace::syscall::exit();
-}
-
-#[panic_handler]
-fn panic(_: &core::panic::PanicInfo) -> ! {
-    loop {}
+extern "C" fn main() {
+    // The runtime handles _start, exit, and panic.
 }