Stop npm oracle trees symlinking into a cycle (#582)

The randomized npm-crawler oracle generator gave some pnpm store
entries a node_modules symlink to nm_dirs.first(), which is usually
the importer node_modules that holds the store. Both crawlers follow
an entry's node_modules link, so they walked
.pnpm/<e>/node_modules/.pnpm/<e>/... until the OS refused the path.
Both treat every I/O error as an empty dir, so the oracle comparison
came down to where each walker gave up, not what it found. Seeds 5,
7, 12 and 41 drew this shape; seed 12 failed CI once with the
sequential walker stopping 6 levels short of the new one.

Point the link at a fresh node_modules outside the tree instead, the
same way the vlt store case and the symlinked-scope case already do.
The followed-symlink shape is still generated (16 of 64 seeds), and
the two oracle tests run about 30% faster.

Co-authored-by: Claude <noreply@anthropic.com>

Author: mikolalysenko

crates/socket-patch-core/src/crawlers/npm_crawler/oracle.rs

@@ -1336,12 +1336,18 @@ mod tests {
                         let _ = std::fs::write(entry.join("node_modules"), "file");
                     }
                     76..=79 => {
-                        // An entry whose node_modules is a symlink.
-                        if let Some(target) = self.nm_dirs.first().cloned() {
-                            let entry = store.join(self.store_entry_name(&name, &version));
-                            let _ = std::fs::create_dir_all(&entry);
-                            Self::symlink(&target, &entry.join("node_modules"));
-                        }
+                        // An entry whose node_modules is a symlink (to a
+                        // dir outside the tree, so the followed walk cannot
+                        // cycle: a link back to an ancestor `node_modules`
+                        // is walked until the OS refuses the path, which
+                        // compares where two walkers gave up, not what
+                        // they found).
+                        let entry = store.join(self.store_entry_name(&name, &version));
+                        let _ = std::fs::create_dir_all(&entry);
+                        let id = self.uniq();
+                        let elsewhere = self.scratch.join(format!("pnpm-nm{id}"));
+                        self.package_json(&elsewhere.join(&name), &name, &version);
+                        Self::symlink(&elsewhere, &entry.join("node_modules"));
                     }
                     80..=83 => {
                         // The entry itself is a symlink (skipped).