refactor: organize workflow/src into 6 module folders

Move 34 flat modules into cas/, registry/, bundle/, extract/, engine/, util/.
Move gc.ts to engine/ (was in cas/) to avoid cas→engine reverse dependency.
Dependency direction: util ← cas ← extract ← engine, util ← registry ← bundle.
No logic changes — only file locations and import paths.

Refs #102

packages/workflow/__tests__/thread-pause-gate.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { createThreadPauseGate } from "../src/thread-pause-gate.js";
+import { createThreadPauseGate } from "../src/engine/thread-pause-gate.js";
 
 describe("createThreadPauseGate", () => {
   test("pause blocks awaitAfterYield until resume", async () => {