Add S3 VFS backend: VfsBackend impl for S3-compatible storage

- S3Vfs with all 15 VfsBackend methods via rusty-s3 + ureq
- S3VfsFile for buffered writes + ranged reads
- AWS Signature V4 pre-signed URLs (rusty-s3)
- ListObjectsV2 for directory listing (prefix + delimiter)
- Path-style URL mapping (/path to bucket/key)

markbase-core/src/vfs/mod.rs

@@ -1,5 +1,6 @@
 pub mod open_flags;
 pub mod local_fs;
+pub mod s3_fs;
 pub mod util;
 
 use std::path::{Path, PathBuf};