feat(v4-10): side_channel toggles in train dropdown → extras.side_channels_observed

Closes V4-10 / cppmega-mlx-zfh. Previously deferred for needing
net-new SideChannelsTab; this lands the minimum viable version via
train-dropdown checkboxes — same surface as train-num-steps.

TopBar:
  - new state scDocIds / scTokenIds; checkboxes
    train-side-channel-doc_ids + train-side-channel-token_ids inside
    the run-pipeline dropdown next to train-num-steps
  - onRunPipeline opts gain side_channels: string[] list of toggled names

App.tsx:
  - handleRunPipeline forwards as stage_options.train.side_channels =
    {name: synthetic_int_list(8 ints)} per toggled name

Backend (stages.py):
  - stage_train reads opts.side_channels dict; for any non-empty list
    value records the key in extras.side_channels_observed[]
  - actual per-channel forward routing is v5+ work — this is the
    observation probe

E2E 24_side_channels.spec.ts:
  - doc_ids only → ["doc_ids"]
  - both toggles → 2 entries
  - none → empty array

3/3 green; 29 stage_train pytest + 11 TopBar vitest still pass.

Author: apstenku123

vbgui/e2e/scenarios/24_side_channels.spec.ts

@@ -0,0 +1,69 @@
+// V4-10: side_channels toggle in train dropdown reaches stage_train
+// → extras.side_channels_observed lists the toggled names.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+
+test("V4-10: doc_ids toggle reaches stage_train side_channels_observed",
+  async ({ page }) => {
+    test.setTimeout(60_000);
+    await gotoApp(page);
+    await selectPreset(page, "llama3_8b");
+
+    await page.getByTestId("run-pipeline-toggle").click();
+    await page.getByTestId("train-side-channel-doc_ids").check();
+    await page.getByTestId("run-pipeline-train").click();
+
+    const modal = page.getByTestId("run-result-modal");
+    await modal.waitFor({ timeout: 60_000 });
+    await page.getByTestId("run-result-expand-train").click();
+
+    // Array rendered with one entry
+    const sc0 = await page.getByTestId(
+      "run-result-extras-train-side_channels_observed-0").textContent();
+    expect(sc0?.trim()).toBe("doc_ids");
+
+    await closeModal(page);
+  });
+
+test("V4-10: both toggles enabled → both observed", async ({ page }) => {
+  test.setTimeout(60_000);
+  await gotoApp(page);
+  await selectPreset(page, "llama3_8b");
+
+  await page.getByTestId("run-pipeline-toggle").click();
+  await page.getByTestId("train-side-channel-doc_ids").check();
+  await page.getByTestId("train-side-channel-token_ids").check();
+  await page.getByTestId("run-pipeline-train").click();
+
+  const modal = page.getByTestId("run-result-modal");
+  await modal.waitFor({ timeout: 60_000 });
+  await page.getByTestId("run-result-expand-train").click();
+
+  const items = page.locator(
+    "[data-testid^='run-result-extras-train-side_channels_observed-']");
+  const count = await items.count();
+  expect(count).toBe(2);
+
+  await closeModal(page);
+});
+
+test("V4-10: no toggle → empty observed list", async ({ page }) => {
+  test.setTimeout(60_000);
+  await gotoApp(page);
+  await selectPreset(page, "llama3_8b");
+
+  await page.getByTestId("run-pipeline-toggle").click();
+  await page.getByTestId("run-pipeline-train").click();
+
+  const modal = page.getByTestId("run-result-modal");
+  await modal.waitFor({ timeout: 60_000 });
+  await page.getByTestId("run-result-expand-train").click();
+
+  const items = page.locator(
+    "[data-testid^='run-result-extras-train-side_channels_observed-']");
+  const count = await items.count();
+  expect(count).toBe(0);
+
+  await closeModal(page);
+});