Function check 

pub fn check(
    lang: &str,
    code: &str,
) -> Result<Vec<DiagramWarning>, MermaidError>

Validate diagram without rendering output.

Parses the input and runs layout (for cycle detection) but skips the expensive grid rendering. Use this for validation-only workflows.

Note: This only catches structural warnings (e.g. cycles). Width-dependent warnings like LabelDropped depend on max_width which is only applied during rendering — use render() if you need those.

Dispatches by lang: "d2" → D2 parser, anything else → Mermaid auto-detect.

Example

use graphs_tui::check;

let warnings = check("mermaid", "flowchart LR\nA --> B\nB --> A").unwrap();
assert!(!warnings.is_empty()); // cycle detected