…to-expand

- Add `body()` / `bodies()` for extracting section content without header
- Add `by_level(sections, l)` to filter sections by heading level (scalar or range)
- Add `section(md_nodes, pattern)` shorthand for sections | title_contains
- Auto-expand section dicts to Markdown nodes in CLI output (no collect() needed)
- Add `collect_markdown_nodes`, `expand_typed_dict`, `runtime_value_to_nodes` in cli.rs
- Add integration tests for auto-expand, no-match empty output, and bodies pipe
- Add module tests for section pattern, no-match, bodies, and by_level variants
- Update example.md with -A flag, import+nodes, include, and code usage patterns
- Update README.md with section usage examples including by_level range pattern

Add `is_typed_dict()` helper that checks if a dict is expandable
without doing the actual node expansion. Replace the `expand_typed_dict().is_some()`
call in `has_expandable` detection with the lighter-weight check.