feat: add run() and binary entry points for all servers and init

Every server now has lib.rs (testable) + main.rs (deployable binary).
Binary is 3 lines: extern crate userspace_rt + call run().
Hardware servers use feature flags for adapter selection.
9 RISC-V binaries ready for the disk image.

crates/servers/session-server/src/main.rs

@@ -0,0 +1,9 @@
+#![no_std]
+#![no_main]
+
+extern crate userspace_rt;
+
+#[unsafe(no_mangle)]
+extern "C" fn main() {
+    session_server::run();
+}