Implement NFS Support stub (Phase 11 P0 #3)

NFS Support Features:
- nfs_server.rs: NFSv3 server stub
- nfs_server CLI tool: Port 2049, export directory
- nfsserve crate dependency (v0.11.0)

Implementation Status:
- NfsVfsServer: Placeholder implementation
- NfsConfig: Configuration struct
- CLI: nfs-server command with --port, --root, --share-name

Technical Details:
- nfsserve crate provides NFSFileSystem trait
- NFSFileSystem requires 14 async methods
- Current implementation is stub (pending API study)

Build: ✅ markbase-core + nfs feature
Tests: 495 markbase-core (without nfs feature)

Note: Full NFS server implementation requires studying nfsserve crate API
(expected time: 2-3 days for 500 lines)

markbase-core/Cargo.toml

@@ -89,12 +89,16 @@ tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 # === LDAP Authentication (Phase 2) ===
 ldap3 = { version = "0.11", optional = true }  # Async LDAP client (compatible with AD + OpenLDAP)
 
+# === NFS Server (Phase 11) ===
+nfsserve = { version = "0.11", optional = true }  # NFSv3/NFSv4 server implementation
+
 [features]
 default = []                          # 默认不启用可选格式
 optional-formats = ["unrar", "xz2", "sevenz-rust"]  # 争议格式可选启用
 smb-server = ["dep:smb-server"]       # SMB server feature flag
 async-vfs = ["dep:reqwest"]           # Async VfsBackend trait + native async S3
 ldap = ["dep:ldap3"]                  # LDAP authentication provider
+nfs = ["dep:nfsserve"]                # NFSv3/NFSv4 server feature flag
 
 [dev-dependencies]
 # tempfile moved to dependencies (needed for archive extraction)