feat(math): emit data-mx-maths on send for cross-client LaTeX

Math rendering already shipped (both the $…$ shorthand and the spec
data-mx-maths form render incoming). But the composer emitted no math, so a
Lotus user's $E=mc^2$ went out as raw text — it rendered on Lotus (both sides
parse $…$) but showed as literal text on Element and other clients.

toMatrixCustomHTML now converts $…$/$$…$$ to
  <span|div data-mx-maths="LATEX"><code>LATEX</code></span|div>
(spec CS-API §11.5), reusing the existing splitMathSegments parser. Math is
extracted BEFORE markdown so LaTeX (_, *, \, {}) isn't mangled, and the emitted
span survives the block-markdown pass via the existing ignoreHTMLParseInlineMD
HTML-tag guard. A new allowMath opt threads through the top-level call sites;
code-line/code-block paths use empty opts so math is off inside code. The plain
body keeps literal $…$ as the fallback.

Scope: inline $…$ + single-line $$…$$. Multi-line block $$ (spans editor
paragraph nodes) deferred — still renders on Lotus via the plain-body path.

New output.test.ts (8 cases): span/div emission, escaping, markdown-bypass,
currency non-match, code-mark + code-block exclusion. 738 tests pass.

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

src/app/features/room/RoomInput.tsx

@@ -514,6 +514,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
           allowTextFormatting: true,
           allowBlockMarkdown: isMarkdown,
           allowInlineMarkdown: isMarkdown,
+          allowMath: true,
         }),
       );
       let msgType = MsgType.Text;
@@ -616,6 +617,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
           allowTextFormatting: true,
           allowBlockMarkdown: isMarkdown,
           allowInlineMarkdown: isMarkdown,
+          allowMath: true,
         }),
       );
       if (plainText === '') return null;

src/app/features/room/message/MessageEditor.tsx

@@ -117,6 +117,7 @@ export const MessageEditor = as<'div', MessageEditorProps>(
             allowTextFormatting: true,
             allowBlockMarkdown: isMarkdown,
             allowInlineMarkdown: isMarkdown,
+            allowMath: true,
           }),
         );