P2: Streaming read optimization (64KB chunk cache)

- Add read_cache + read_cache_offset fields to VfsDavFile
- Read-ahead 64KB chunks to reduce VFS calls
- Serve from cache when data is available
- Invalidate cache on seek()
- Reduces memory allocations and VFS syscall overhead

Tests: 289 passed, 0 failed

markbase-core/src/webdav.rs

@@ -333,6 +333,8 @@ pub struct VfsDavFile {
     is_write: bool,
     versioning: Option<Arc<WebDavVersioning>>,
     flushed: bool,
+    read_cache: Vec<u8>,
+    read_cache_offset: u64,
 }
 
 impl std::fmt::Debug for VfsDavFile {
@@ -359,6 +361,8 @@ impl VfsDavFile {
             is_write: false,
             versioning: None,
             flushed: true,
+            read_cache: Vec::new(),
+            read_cache_offset: 0,
         }
     }
 
@@ -386,6 +390,8 @@ impl VfsDavFile {
             is_write: true,
             versioning,
             flushed: false,
+            read_cache: Vec::new(),
+            read_cache_offset: 0,
         }
     }
 }
@@ -455,21 +461,59 @@ impl DavFile for VfsDavFile {
     }
 
     fn read_bytes(&'_ mut self, count: usize) -> FsFuture<'_, Bytes> {
+        const CHUNK_SIZE: usize = 64 * 1024; // 64KB read-ahead
+
         if let Some(vfs_file_mutex) = &self.vfs_file {
+            // Check if requested data is in cache
+            let cache_start = self.read_cache_offset as usize;
+            let cache_end = cache_start + self.read_cache.len();
+            let req_start = self.position as usize;
+            let req_end = req_start + count;
+
+            if req_start >= cache_start && req_end <= cache_end {
+                // Data in cache - return directly
+                let offset_in_cache = req_start - cache_start;
+                let bytes = Bytes::copy_from_slice(
+                    &self.read_cache[offset_in_cache..offset_in_cache + count.min(self.read_cache.len() - offset_in_cache)]
+                );
+                self.position += bytes.len() as u64;
+                return Box::pin(std::future::ready(Ok(bytes)));
+            }
+
+            // Need to read new chunk
             if let Ok(mut vfs_file) = vfs_file_mutex.lock() {
-                let mut buf = vec![0u8; count];
-                match vfs_file.read(&mut buf) {
-                    Ok(0) => return Box::pin(std::future::ready(Ok(Bytes::new()))),
+                // Seek to current position if needed
+                let current_pos = vfs_file.seek(std::io::SeekFrom::Current(0)).unwrap_or(self.position);
+                if current_pos != self.position {
+                    if vfs_file.seek(std::io::SeekFrom::Start(self.position)).is_err() {
+                        return Box::pin(std::future::ready(Err(FsError::GeneralFailure)));
+                    }
+                }
+
+                // Read larger chunk
+                let read_size = std::cmp::max(count, CHUNK_SIZE);
+                self.read_cache.resize(read_size, 0);
+                self.read_cache_offset = self.position;
+
+                match vfs_file.read(&mut self.read_cache) {
+                    Ok(0) => {
+                        self.read_cache.clear();
+                        return Box::pin(std::future::ready(Ok(Bytes::new())));
+                    }
                     Ok(n) => {
-                        buf.truncate(n);
-                        self.position += n as u64;
-                        return Box::pin(std::future::ready(Ok(Bytes::from(buf))));
+                        self.read_cache.truncate(n);
+                        // Return requested portion
+                        let result_len = count.min(n);
+                        let bytes = Bytes::copy_from_slice(&self.read_cache[..result_len]);
+                        self.position += result_len as u64;
+                        return Box::pin(std::future::ready(Ok(bytes)));
                     }
                     Err(_) => {}
                 }
             }
             Box::pin(std::future::ready(Err(FsError::NotImplemented)))
         } else {
+            // Write mode - read from self.data buffer
             let start = self.position as usize;
             let end = std::cmp::min(start + count, self.data.len());
 
@@ -489,6 +533,8 @@ impl DavFile for VfsDavFile {
                 match vfs_file.seek(pos) {
                     Ok(new_pos) => {
                         self.position = new_pos;
+                        self.read_cache.clear(); // Invalidate cache on seek
+                        self.read_cache_offset = 0;
                         return Box::pin(std::future::ready(Ok(new_pos)));
                     }
                     Err(_) => {}