docs: mark P4-4 Math/LaTeX done + note outgoing data-mx-maths interop

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_FEATURES.md

@@ -706,8 +706,9 @@ KaTeX-rendered math in messages, two paths:
 
 - **Spec path (CS-API §11.5):** `<span/div data-mx-maths="…">` in `formatted_body` renders the attribute's LaTeX (block for div, inline for span); on render failure the element's child fallback content shows instead
 - **Plain-text path:** `$…$` (inline) and `$$…$$` (block) with conservative rules — escape-aware (`\$`), currency-guarded (`$5 and $10` stays text), never inside `code`/`pre`
+- **Outgoing interop:** on send, the composer converts `$…$`/`$$…$$` to spec `<span/div data-mx-maths>` HTML in `formatted_body` (extracted before markdown so LaTeX isn't mangled; off inside code), so math renders on Element and every other client — not just Lotus. The plain `body` keeps literal `$…$` as the fallback
 - KaTeX + its CSS load lazily on first math encountered — zero cost to the main bundle
-- Files: `src/app/utils/mathParse.ts` (+14 tests), `components/math/KaTeX.tsx`, `plugins/react-custom-html-parser.tsx`
+- Files: `src/app/utils/mathParse.ts` (+14 tests), `components/math/KaTeX.tsx`, `plugins/react-custom-html-parser.tsx` (render), `components/editor/output.ts` (+ `output.test.ts`, outgoing)
 
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