fix: final-review wave (CSRF guard, fail-closed owner bootstrap, streamed zips, e2e visibility suite)

- Critical 1 (CSRF): core wrap refuses every POST/PUT/PATCH/DELETE whose Origin
  (Referer as fallback) is not publicUrl's origin or STC_ALLOWED_ORIGINS, and
  any body that is not application/json (multipart only on declared upload
  routes). Better Auth routes go through the same guard; only declared
  identity-provider callbacks (SAML ACS/SLO, OAuth callbacks) are exempt.
  Better Auth trustedOrigins use the same list. Session cookie stays Lax
  (Strict breaks OAuth/OIDC/SAML returns); documented.
- Important 4 / learner minor 4: the first-owner claim and the role row are
  one transaction guarded by "no owner yet"; only a unique violation means
  learner; any other error fails the sign-up with 500 and removes the
  half-created user. Serialised per DB handle (Bun's SQLite connection).
- Important 2 / learner minor 12: code zips stream entry by entry (large
  files stored in 256 KB chunks with a data descriptor and incremental
  CRC-32), so memory stays at one chunk; the cap is now 1 GiB (ZIP32).
- Important 3 / learner minor 10: the e2e runs the production wiring
  (productFeatures order, AI drafts through the real sink) against a
  restricted course and asserts 404/401 for learner and anonymous on
  course, lesson, media, cover, code, zip, notes, progress, attempts, AI
  chat, library, search and AI status; a real Range request (206, 416);
  forged writes (foreign Origin, text/plain, no Origin) are 403 and the MCP
  test command never runs.
- Minors 7-9: compose passes every documented variable; the health check
  uses STC_PORT; productFeatures checks the studio-before-AI order.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/feature-studio/test/security.test.ts

@@ -202,7 +202,7 @@ describe("I2: uploads", () => {
     const ct = new Response(fd).headers.get("content-type")!;
     const bytes = new Uint8Array(await new Response(fd).arrayBuffer());
     const stream = new ReadableStream({ start(c) { for (let i = 0; i < bytes.length; i += 65536) c.enqueue(bytes.slice(i, i + 65536)); c.close(); } });
-    const r = await env.app.fetch(new Request("http://x/api/studio/courses/c1/upload", { method: "POST", headers: { "x-user": "ada", "content-type": ct }, body: stream, duplex: "half" } as RequestInit));
+    const r = await env.app.fetch(new Request("http://x/api/studio/courses/c1/upload", { method: "POST", headers: { "x-user": "ada", origin: "http://x", "content-type": ct }, body: stream, duplex: "half" } as RequestInit));
     expect(r.status).toBe(413);
     expect(existsSync(join(env.dataDir, "worktrees/c1/drafts/u-ada/big.zip"))).toBe(false);
     expect((await upload("small.zip", new Uint8Array(1000))).status).toBe(200);