Add prefetch cache for VFS to optimize file reads

- Introduced a `PrefetchCache` structure to store pre-read file contents and canonical paths, enhancing the speed of initial reads during snapshot building.
- Implemented parallel file reading using `rayon` and `walkdir` to populate the cache efficiently.
- Updated the `VfsInner` struct to include the prefetch cache and modified read methods to utilize it, reducing I/O operations.
- Added functions to set and clear the prefetch cache in the `Vfs` API, improving memory management during live operations.
- Enhanced tests to validate the functionality of the prefetch cache, ensuring correct behavior during file reads and canonicalization.

These changes aim to significantly improve the performance of the VFS by minimizing file access times during initial operations.

src/snapshot_middleware/dir.rs

@@ -49,19 +49,41 @@ pub fn snapshot_dir_no_meta(
             .all(|rule| rule.passes(child.path()))
     };
 
-    let mut snapshot_children = Vec::new();
-
-    for entry in vfs.read_dir(path)? {
-        let entry = entry?;
-
-        if !passes_filter_rules(&entry) {
-            continue;
+    let snapshot_children = if std::env::var("ATLAS_SEQUENTIAL").is_ok() {
+        let mut children = Vec::new();
+        for entry in vfs.read_dir(path)? {
+            let entry = entry?;
+            if !passes_filter_rules(&entry) {
+                continue;
+            }
+            if let Some(child_snapshot) = snapshot_from_vfs(context, vfs, entry.path())? {
+                children.push(child_snapshot);
+            }
         }
+        children
+    } else {
+        use rayon::prelude::*;
 
-        if let Some(child_snapshot) = snapshot_from_vfs(context, vfs, entry.path())? {
-            snapshot_children.push(child_snapshot);
+        let entries: Vec<_> = vfs.read_dir(path)?.filter_map(|e| e.ok()).collect();
+
+        let results: Vec<anyhow::Result<Option<InstanceSnapshot>>> = entries
+            .par_iter()
+            .map(|entry| {
+                if !passes_filter_rules(entry) {
+                    return Ok(None);
+                }
+                snapshot_from_vfs(context, vfs, entry.path())
+            })
+            .collect();
+
+        let mut children = Vec::new();
+        for result in results {
+            if let Some(snapshot) = result? {
+                children.push(snapshot);
+            }
         }
-    }
+        children
+    };
 
     let normalized_path = vfs.canonicalize(path)?;
     let relevant_paths = vec![