feat(helper/stream): add browser-compatible Writable interface

Define Writable interface, ConsoleWritable (console.log), and
StderrWritable (console.error) as browser-safe stream abstractions.
Export DEFAULT_STREAM backed by StderrWritable to match Python's
sys.stderr default.

src/helper/stream.ts

@@ -1,2 +1,33 @@
+/**
+ * Minimal writable stream interface for browser compatibility.
+ *
+ * This abstracts over Node.js streams and browser console output,
+ * providing a common write target for handlers.
+ */
+export interface Writable {
+    write(data: string): void;
+}
 
-export type MillisecondsSinceUnixEpoch = number;
+/**
+ * A Writable backed by console.log.
+ */
+export class ConsoleWritable implements Writable {
+    write(data: string): void {
+        console.log(data);
+    }
+}
+
+/**
+ * A Writable backed by console.error (stderr equivalent).
+ */
+export class StderrWritable implements Writable {
+    write(data: string): void {
+        console.error(data);
+    }
+}
+
+/**
+ * Default output stream. Uses console.error to match Python's default
+ * of writing to sys.stderr.
+ */
+export const DEFAULT_STREAM: Writable = new StderrWritable();