Function apply_fixes 

pub fn apply_fixes(
    plan: &FixPlan,
    lang: &dyn LanguageFixProvider,
    project_root: &Path,
) -> Result<FixResult>

Apply a fix plan to disk.

lang provides language-specific post-processing (e.g., import deduplication). After all files are written, calls lang.post_apply() for ecosystem-specific steps (e.g., npm install after package.json changes).