Function render_error 

pub fn render_error(err: &TixError) -> String

Render any TixError as one rustc-style block.

The single boundary renderer for errors. The CLI prints it to stderr and MCP returns it as the isError text, so the two surfaces cannot drift: their parity is not maintained by two similar format!s but by there being one function. TixError::Refused already carries a rendered block and is returned verbatim; every other variant is rendered from its Display text under a stable TixError::diagnostic_id.