feat(mcp): stdio transport adapter (T10.4, FR-MCP-6)

serve_once() runs one MCP session over the current process's stdin/
stdout to completion -- the entry point main.rs's --mcp-stdio flag calls
into (next commit). No bearer-token check here: spawning this process
at all requires the same local-user privilege as any other command, so
process-spawn capability is the trust boundary for stdio, not a header.

src-tauri/src/mcp/stdio_transport.rs

@@ -0,0 +1,31 @@
+//! stdio transport (FR-MCP-6) — "a thin adapter the agent spawns". A coding
+//! agent's MCP client config spawns `whispassist.exe --mcp-stdio` and talks
+//! JSON-RPC over that child process's stdin/stdout; `main.rs` checks for that
+//! flag before building the Tauri window and calls `serve_once` here instead.
+//!
+//! There is no bearer-token header to check here (unlike HTTP): the ability
+//! to spawn this process at all already requires the same OS-level privilege
+//! as running any other local command as the signed-in user, so process-spawn
+//! capability is the trust boundary for stdio, same as other local-only MCP
+//! servers. `set_mcp_enabled` still mints/stores a token (`mcp::token`) for
+//! parity with the HTTP transport and in case a future stdio client wants to
+//! pass it, but this transport does not require presenting it.
+
+use crate::mcp::handler::WaMcpHandler;
+use crate::mcp::McpError;
+use rmcp::ServiceExt;
+
+/// Serves one MCP session over the current process's stdin/stdout until the
+/// peer disconnects, then returns.
+pub async fn serve_once(handler: WaMcpHandler) -> Result<(), McpError> {
+    let transport = rmcp::transport::io::stdio();
+    let running = handler
+        .serve(transport)
+        .await
+        .map_err(|e| McpError::Server(e.to_string()))?;
+    running
+        .waiting()
+        .await
+        .map_err(|e| McpError::Server(e.to_string()))?;
+    Ok(())
+}