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Task 7 of the native-surfaces migration plan. Pure previewAnnotations.ts (annotateSource + cowritingMarkdownItPlugin) reuses trackChangesModel.ts's sentinel discipline (#33/#47), generalized to a 3-way ins-claude/ins-human/del tag (exported injectSentinels/sentinelsToSpans, colorByAuthor unaffected, 84/84 existing tests pass unmodified). Host hook in extension.ts wires registry/diffView/attribution/proposals/config into the plugin, exposed via CowritingApi.extendMarkdownIt + a previewAnnotationHost test seam. cowriting.annotations setting + toggleAnnotations command/menu. Q4 mermaid (previewScripts + options.highlight fence override) implemented per the proven bierner.markdown-mermaid pattern; intra-diagram diff augmentation scoped out (unverifiable via the structural E2E harness) — see task-7-report.md. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>