Refactor: Introduce system library

- Add `rtl/system.myc` as the system library, containing core utilities
  like `while` and `cache`.
- Remove the duplicate `prelude.myc` from `src/ast/rtl/`.
- Modify `Environment::collect_dependencies` to implicitly add `#use
  system` when necessary.
- Update `Environment::with_cwd` to also add the `rtl` subdirectory to
  search paths.
- Add `target/` to `.geminiignore` to exclude build artifacts.
- Adjust example `sma.myc` to use the new `cache` macro and reduce
  sample data size.
- Update `rtl/sma.myc` to correctly initialize the `history` series with
  its `length`.
- Add `preload_dependencies` calls to `dump_ast` and `run_script` for
  proper module loading.

.geminiignore

@@ -1,2 +1,3 @@
 *.snap
 *.snap.new
+target/