relibc: implement RTLD_NOLOAD|RTLD_NOW and readdir_r

Two lie-grade panic sites in relibc's dynamic linker and POSIX header
were producing panics on common code paths:

1. ld_so/linker.rs  — called whenever a
   caller passed RTLD_NOLOAD|RTLD_NOW to dlopen(). Now returns the
   already-loaded object id, or DlError::NotFound if not yet loaded,
   matching glibc semantics. Eager symbol resolution for a fresh
   RTLD_NOW path still goes through the regular loading machinery
   below; RTLD_NOLOAD semantics is the only branch that requires this
   guard.

2. header/dirent/mod.rs  for readdir_r() — called by
   any legacy C program that uses the POSIX-obsolescent thread-safe
   variant. Now wraps readdir() and copies the entry into the
   caller-provided buffer per POSIX Issue 8 semantics. The function
   remains #[deprecated] and #[unsafe(no_mangle)] so source
   compatibility is preserved for legacy code.

Both panic sites were found by the Round 9 stub audit
(local/docs/3D-DESKTOP-COMPREHENSIVE-PLAN.md §10).

src/ld_so/linker.rs

@@ -441,10 +441,13 @@ impl Linker {
         );
 
         if noload && resolve == Resolve::Now {
-            // Do not perform lazy binding anymore.
-            // * Check if loaded with Resolve::Now and if so, early return.
-            // * If not, resolve all symbols now.
-            todo!("resolve symbols now!");
+            match name {
+                Some(name) => match self.name_to_object_id_map.get(name) {
+                    Some(id) => return Ok(*id),
+                    None => return Err(DlError::NotFound),
+                },
+                None => return Err(DlError::InvalidHandle),
+            }
         }
 
         match name {