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-core/src/error.rs

@@ -55,6 +55,13 @@ pub enum ErrorCode {
     NoMarketData,
 }
 
+/// The stable wire form frontends emit, e.g. `SYMBOLNOTFOUND`.
+impl std::fmt::Display for ErrorCode {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&format!("{self:?}").to_uppercase())
+    }
+}
+
 impl IdxError {
     pub fn code(&self) -> ErrorCode {
         match self {