Extract data_server into external data-server crate

The data_server module had no dependency on the rest of myc (only
chrono/regex/zip), so it was moved verbatim into a standalone leaf
crate at its own Gitea repo and is now consumed as a git dependency.
A 'pub use ::data_server;' re-export in src/ast/mod.rs keeps all
existing myc::ast::data_server::… paths valid; no call sites changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ast/rtl/data_streams.rs

@@ -1,6 +1,6 @@
 //! RTL registration for file-based market data streams.
 //!
-//! Bridges the `Arc`-based [`DataServer`] cache with the `Rc`-based VM world
+//! Bridges the `Arc`-based [`::data_server::DataServer`] cache with the `Rc`-based VM world
 //! by registering generator closures that read pre-parsed chunks and convert
 //! them into `Value::Record` items for `RootStream::tick()`.