fix: address Phase 1 code-quality review findings

- Generator feature templates emit ctx-arg signatures + updated checklist
- signUpUseCase test for bus=undefined path
- di-explainer.html: bind-dev-seed no-arg → ctx
- data-flow-explainer.html: tradeoff card → ctx
- BindContextBase no longer exported
- CLAUDE.md binder bullet: drop vestigial cast guidance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/auth/src/application/use-cases/sign-up.use-case.test.ts

@@ -60,6 +60,21 @@ describe("signUpUseCase", () => {
     );
   });
 
+  it("works without an event bus (welcome email skipped silently)", async () => {
+    const users = new MockUsersRepository([]);
+    const auth = new MockAuthenticationService(users);
+    const useCase = signUpUseCase(users, auth, undefined);
+
+    const result = await useCase({
+      username: "frank",
+      password: "secret_password",
+      confirmPassword: "secret_password",
+    });
+
+    expect(result.session.userId).toBeTruthy();
+    expect(result.cookie.name).toBe("session");
+  });
+
   it("does NOT publish when sign-up fails (username taken)", async () => {
     const users = new MockUsersRepository([]);
     const auth = new MockAuthenticationService(users);