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/src/cli/tools/mod.rs

@@ -1,6 +1,8 @@
 pub mod render;
 pub mod smb_server;
 pub mod test;
+#[cfg(feature = "nfs")]
+pub mod nfs_server;
 
 use clap::Subcommand;
 
@@ -12,6 +14,8 @@ pub enum ToolsCommands {
     Test(test::TestCommand),
     #[command(flatten)]
     SmbServer(smb_server::SmbServerCommand),
+    #[cfg(feature = "nfs")]
+    Nfs(nfs_server::NfsServerCommand),
 }
 
 pub async fn handle_tools_command(cmd: ToolsCommands) -> anyhow::Result<()> {
@@ -19,6 +23,8 @@ pub async fn handle_tools_command(cmd: ToolsCommands) -> anyhow::Result<()> {
         ToolsCommands::Render(c) => render::handle_render_command(c)?,
         ToolsCommands::Test(c) => test::handle_test_command(c)?,
         ToolsCommands::SmbServer(c) => smb_server::handle_smb_server_command(c).await?,
+        #[cfg(feature = "nfs")]
+        ToolsCommands::Nfs(c) => nfs_server::run_nfs_server(c).await?,
     }
     Ok(())
 }