feat(mcp): add idx-mcp, a stdio MCP server over idx-core

Phase 4 of docs/DESIGN.md. Eleven grouped tools (quotes, history, screen,
company, analysis, compare, shareholders, investor holdings, search,
ownership changes, status); ownership_sync is listed only with
--allow-sync. Live results keep the Fetched envelope, errors use the CLI's
{error, code, message} body, and investor tools on an empty database
point at the sync path.

idx-core gains Period/Interval FromStr, Investor::status, a ticker
filter on changes, and Display for ErrorCode (now used by the CLI too).
CLI output and goldens are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-mcp/src/main.rs

@@ -0,0 +1,51 @@
+//! `idx-mcp`: an MCP server (stdio) over idx-core. See docs/DESIGN.md.
+
+mod params;
+mod server;
+
+use std::process::ExitCode;
+
+use clap::Parser;
+use idx_core::config::CoreConfig;
+use rmcp::ServiceExt;
+
+#[derive(Debug, Parser)]
+#[command(
+    name = "idx-mcp",
+    version,
+    about = "MCP server (stdio) for Indonesian stock market (IDX) data"
+)]
+struct Args {
+    /// Expose the ownership_sync tool, which downloads the KSEI ownership
+    /// snapshot (several MB) into the local database.
+    #[arg(long)]
+    allow_sync: bool,
+}
+
+#[tokio::main]
+async fn main() -> ExitCode {
+    let args = Args::parse();
+    let config = match CoreConfig::load() {
+        Ok(config) => config,
+        Err(err) => {
+            eprintln!("error: {err}");
+            return ExitCode::from(err.exit_code() as u8);
+        }
+    };
+
+    let server = server::IdxServer::new(config, args.allow_sync);
+    let running = match server.serve(rmcp::transport::stdio()).await {
+        Ok(running) => running,
+        Err(err) => {
+            eprintln!("error: {err}");
+            return ExitCode::FAILURE;
+        }
+    };
+    match running.waiting().await {
+        Ok(_) => ExitCode::SUCCESS,
+        Err(err) => {
+            eprintln!("error: {err}");
+            ExitCode::FAILURE
+        }
+    }
+}