kernel: resolve /proc/self/fd/N and /proc/self/stat etc.

Extended /proc/self resolution to handle sub-paths like
/proc/self/fd/0, /proc/self/stat, /proc/self/status, etc.

Previously only /proc/self (the directory itself) was resolved.
Now /proc/self/fd, /proc/self/stat, /proc/self/maps, and all
other sub-paths are resolved by replacing 'self' with the
current PID and delegating to the standard proc_open() parser.

Cross-referenced with Linux 7.1 fs/proc/self.c proc_self_get_link()
which creates a symlink to the current PID directory. This
completes the /proc/self implementation for all sub-entries.

src/scheme/proc.rs

@@ -534,7 +534,7 @@ 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>
+        // /proc/self[/...] → resolve /proc/<current-pid>[/...]
         if operation_name == "proc/self" || operation_name == "self" {
             let pid = context::current().read(token.token()).pid;
             let handle = Handle {
@@ -542,7 +542,26 @@ impl ProcScheme {
                 kind: ContextHandle::ProcDir { pid },
             };
             return new_handle((handle, InternalFlags::POSITIONED), token);
-        };
+        }
+        if operation_name.starts_with("proc/self/") || operation_name.starts_with("self/") {
+            let pid = context::current().read(token.token()).pid;
+            let rest = if operation_name.starts_with("proc/self/") {
+                &operation_name["proc/self/".len()..]
+            } else {
+                &operation_name["self/".len()..]
+            };
+            let resolved = format!("proc/{}/{}", pid, rest);
+            let mut parts = resolved.split('/');
+            let _ = parts.next(); // "proc"
+            let pid = parts.next().and_then(|p| p.parse::<usize>().ok()).unwrap_or(pid);
+            if let Some(kind) = Self::proc_open(&resolved) {
+                let handle = Handle {
+                    context: context::current(),
+                    kind,
+                };
+                return new_handle((handle, InternalFlags::POSITIONED), token);
+            }
+        }
         if let Some(kind) = Self::proc_open(operation_name) {
             let handle = Handle {
                 context: context::current(),