Cap dev-runner output buffering

Co-Authored-By: Paperclip <noreply@paperclip.ing>

scripts/dev-runner-output.mjs

@@ -0,0 +1,53 @@
+const DEFAULT_CAPTURED_OUTPUT_BYTES = 256 * 1024;
+
+export function createCapturedOutputBuffer(maxBytes = DEFAULT_CAPTURED_OUTPUT_BYTES) {
+  const limit = Math.max(1, Math.trunc(maxBytes));
+  const chunks = [];
+  let bufferedBytes = 0;
+  let totalBytes = 0;
+  let truncated = false;
+
+  return {
+    append(chunk) {
+      if (chunk === null || chunk === undefined) return;
+      const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+      if (buffer.length === 0) return;
+
+      chunks.push(buffer);
+      bufferedBytes += buffer.length;
+      totalBytes += buffer.length;
+
+      while (bufferedBytes > limit && chunks.length > 0) {
+        const overflow = bufferedBytes - limit;
+        const head = chunks[0];
+        if (head.length <= overflow) {
+          chunks.shift();
+          bufferedBytes -= head.length;
+          truncated = true;
+          continue;
+        }
+
+        chunks[0] = head.subarray(overflow);
+        bufferedBytes -= overflow;
+        truncated = true;
+      }
+    },
+
+    finish() {
+      const body = Buffer.concat(chunks).toString("utf8");
+      if (!truncated) {
+        return {
+          text: body,
+          truncated,
+          totalBytes,
+        };
+      }
+
+      return {
+        text: `[output truncated to last ${limit} bytes; total ${totalBytes} bytes]\n${body}`,
+        truncated,
+        totalBytes,
+      };
+    },
+  };
+}