Split streaming markdown into memoized blocks for mobile perf (#260)

Instead of re-parsing the entire markdown string on every streaming
token, split at paragraph boundaries (double newlines) while keeping
code fences intact. Each block renders as its own React.memo'd
<Markdown> component so only the last active block re-renders.

packages/app/src/utils/split-markdown-blocks.ts

@@ -0,0 +1,60 @@
+function getFenceDelimiter(line: string) {
+  const match = /^( {0,3})(`{3,}|~{3,})/.exec(line);
+  return match?.[2] ?? null;
+}
+
+export function splitMarkdownBlocks(text: string): string[] {
+  if (text.length === 0) {
+    return [];
+  }
+
+  const blocks: string[] = [];
+  let currentLines: string[] = [];
+  let activeFenceCharacter: "`" | "~" | null = null;
+  let activeFenceLength = 0;
+  let sawBlockSeparator = false;
+
+  for (const line of text.split("\n")) {
+    const isBlankLine = line.trim().length === 0;
+
+    if (!activeFenceCharacter && isBlankLine) {
+      if (currentLines.length > 0) {
+        sawBlockSeparator = true;
+      }
+      continue;
+    }
+
+    if (!activeFenceCharacter && sawBlockSeparator) {
+      blocks.push(currentLines.join("\n"));
+      currentLines = [];
+      sawBlockSeparator = false;
+    }
+
+    currentLines.push(line);
+
+    const fenceDelimiter = getFenceDelimiter(line);
+    if (!fenceDelimiter) {
+      continue;
+    }
+
+    if (!activeFenceCharacter) {
+      activeFenceCharacter = fenceDelimiter[0] as "`" | "~";
+      activeFenceLength = fenceDelimiter.length;
+      continue;
+    }
+
+    if (
+      fenceDelimiter[0] === activeFenceCharacter &&
+      fenceDelimiter.length >= activeFenceLength
+    ) {
+      activeFenceCharacter = null;
+      activeFenceLength = 0;
+    }
+  }
+
+  if (currentLines.length > 0) {
+    blocks.push(currentLines.join("\n"));
+  }
+
+  return blocks.filter((block) => block.length > 0);
+}