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 }

crates/userspace-rt/src/arch/mod.rs

@@ -0,0 +1,2 @@
+#[cfg(all(target_arch = "riscv64", target_os = "none"))]
+mod riscv;

crates/userspace-rt/src/arch/riscv.rs

@@ -0,0 +1,13 @@
+use core::arch::global_asm;
+
+global_asm!(
+    r#"
+.section .text.entry
+.globl _start
+_start:
+    call main
+    li a7, 16
+    ecall
+    j _start
+"#
+);

crates/userspace-rt/src/lib.rs

@@ -0,0 +1,12 @@
+#![no_std]
+
+mod arch;
+
+pub use userspace::syscall;
+pub use userspace::allocator::KosAllocator;
+
+#[cfg(target_os = "none")]
+#[panic_handler]
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    userspace::syscall::exit();
+}