fix(mcp): route tracing output to stderr to prevent JSON-RPC stdio corruption (#470)

The ruvector-mcp binary initializes its tracing subscriber without
specifying a writer, defaulting to stdout. Under the stdio MCP
transport this contaminates the JSON-RPC frame stream with log lines,
causing every @modelcontextprotocol/sdk client to throw a Zod parse
error on the very first frame.

Add .with_writer(std::io::stderr) to both the debug and release
tracing subscriber builders in crates/ruvector-cli/src/mcp_server.rs.

Verified by stdio smoke test: first line of stdout is now a valid
JSON-RPC initialize response with serverInfo.name == "ruvector-mcp",
and tracing output appears exclusively on stderr as required by the
MCP stdio transport spec.

crates/ruvector-cli/src/mcp_server.rs

@@ -48,10 +48,12 @@ async fn main() -> Result<()> {
     // Initialize logging
     if cli.debug {
         tracing_subscriber::fmt()
+            .with_writer(std::io::stderr)
             .with_env_filter("ruvector=debug")
             .init();
     } else {
         tracing_subscriber::fmt()
+            .with_writer(std::io::stderr)
             .with_env_filter("ruvector=info")
             .init();
     }