kernel: implement /proc/self — symlink to current process

Added /proc/self resolution in open_inner(). When /proc/self
is opened, it resolves to /proc/<current-pid> by reading
context::current().pid and returning a ProcDir handle.

Cross-referenced with Linux 7.1 fs/proc/self.c proc_self_get_link()
which creates a symlink inode pointing to the current PID.

This enables 'ls /proc/self', 'cat /proc/self/status', and all
other tools that use /proc/self to access their own process info.

src/scheme/proc.rs

@@ -534,6 +534,15 @@ impl ProcScheme {
         token: &mut CleanLockToken,
     ) -> Result<(usize, InternalFlags)> {
         let operation_name = operation_str.ok_or(Error::new(EINVAL))?;
+        // /proc/self → resolve to /proc/<current-pid>
+        if operation_name == "proc/self" || operation_name == "self" {
+            let pid = context::current().read(token.token()).pid;
+            let handle = Handle {
+                context: context::current(),
+                kind: ContextHandle::ProcDir { pid },
+            };
+            return new_handle((handle, InternalFlags::POSITIONED), token);
+        };
         if let Some(kind) = Self::proc_open(operation_name) {
             let handle = Handle {
                 context: context::current(),