feat(h08): train-probe-text textarea forwards inference_probe_text

Closes V5-G20 from the UI side: TopBar's Train dropdown now exposes
a `train-probe-text` textarea that App.handleRunPipeline forwards into
`stage_options.train.inference_probe_text`. Backend stage_train pairs
it with the active tokenizer to encode real token ids for the pre-vs-
post forward divergence probe (instead of synthetic Gaussian noise).

Tests:
- vbgui vitest TopBar: H08 specs for the textarea + empty-omits-field
  paths — 22/22 TopBar tests passing.
- vbgui e2e 62_probe_text.spec.ts: wire real tokenizer, fill probe
  text, Train → assert extras.inference_probe.real_tokens=true,
  text_len>0, top1_token_drift>=0.
- vbgui e2e V6 suite (55–62): 10/10 passing — no regression.

Author: apstenku123

vbgui/e2e/scenarios/62_probe_text.spec.ts

@@ -0,0 +1,59 @@
+// H08: TopBar train-probe-text textarea threads opts.inference_probe_text
+// into stage_train (V5-G20). Backend pairs the text with the active
+// tokenizer to encode real token ids for the pre-vs-post forward
+// divergence probe, replacing the synthetic random Gaussian input.
+//
+// Assertions:
+//   - extras.inference_probe.real_tokens === true
+//   - extras.inference_probe.text_len > 0
+//   - extras.inference_probe.top1_token_drift >= 0 (number, not "null")
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, clickTab, closeModal } from "../fixtures";
+import { loadMatrix } from "../utils/matrix";
+
+const MATRIX = loadMatrix();
+const REAL_TOKENIZER = MATRIX.tokenizers.T2_gpt2_small.path;
+
+test("H08: train-probe-text → extras.inference_probe.real_tokens=true",
+  async ({ page }) => {
+    test.setTimeout(120_000);
+    await gotoApp(page);
+    await selectPreset(page, "llama3_8b");
+
+    // Wire tokenizer so the backend can encode the probe text.
+    await clickTab(page, "tokenizer");
+    await page.getByTestId("tokenizer-playground").waitFor();
+    await page.getByTestId("add-panel").click();
+    await page.getByTestId("tokenizer-source-0").fill(REAL_TOKENIZER);
+    await page.getByTestId("tokenizer-encode-0").click();
+    await page.getByTestId("tokenizer-metrics-0").waitFor({ timeout: 8_000 });
+    await page.getByTestId("tokenizer-use-for-train-0").click();
+
+    // Drive Train with probe text.
+    await clickTab(page, "canvas");
+    await page.getByTestId("run-pipeline-toggle").click();
+    await page.getByTestId("train-num-steps").fill("2");
+    await page.getByTestId("train-probe-text")
+      .fill("hello world this is a probe sentence");
+    await page.getByTestId("run-pipeline-train").click();
+    await page.getByTestId("run-result-modal").waitFor({ timeout: 60_000 });
+    await page.getByTestId("run-result-expand-train").click();
+    await page.getByTestId("run-result-extras-row-train").waitFor();
+
+    const real = await page.getByTestId(
+      "run-result-extras-train-inference_probe-real_tokens").textContent();
+    expect(real?.trim().toLowerCase()).toBe("true");
+
+    const textLen = parseInt(await page.getByTestId(
+      "run-result-extras-train-inference_probe-text_len")
+      .textContent() ?? "0", 10);
+    expect(textLen).toBeGreaterThan(0);
+
+    const drift = await page.getByTestId(
+      "run-result-extras-train-inference_probe-top1_token_drift").textContent();
+    expect(drift?.trim()).not.toBe("null");
+    expect(parseFloat(drift!.trim())).toBeGreaterThanOrEqual(0);
+
+    await closeModal(page);
+  });

vbgui/src/App.tsx

@@ -302,6 +302,7 @@ export function App(): JSX.Element {
     opts?: { num_steps?: number; warm_start?: boolean;
       checkpoint_save_path?: string;
       checkpoint_load_path?: string;
+      inference_probe_text?: string;
     },
   ) => {
     const snap = wireSpecRef.current;
@@ -338,6 +339,12 @@ export function App(): JSX.Element {
       if (opts?.checkpoint_load_path) {
         trainOpts.checkpoint_load_path = opts.checkpoint_load_path;
       }
+      // H08: forward inference probe text (G20). Backend pairs it with
+      // tokenizer_path (also forwarded above) to encode real tokens for
+      // the pre-vs-post forward divergence probe.
+      if (opts?.inference_probe_text) {
+        trainOpts.inference_probe_text = opts.inference_probe_text;
+      }
       if (trainParquetPath) trainOpts.parquet_path = trainParquetPath;
       if (trainTokenizerPath) trainOpts.tokenizer_path = trainTokenizerPath;
       // Forward SideChannelsTab train selection as synthetic int lists for the

vbgui/src/components/TopBar.tsx

@@ -17,6 +17,7 @@ export interface TopBarProps {
     opts?: { num_steps?: number; warm_start?: boolean;
       checkpoint_save_path?: string;
       checkpoint_load_path?: string;
+      inference_probe_text?: string;
     }) => void;
   /** H02: toggle callbacks. */
   onMixedPrecisionChange?: (enabled: boolean) => void;
@@ -45,6 +46,7 @@ export function TopBar(p: TopBarProps): JSX.Element {
   const [warmStart, setWarmStart] = useState<boolean>(false);
   const [ckptSavePath, setCkptSavePath] = useState<string>("");
   const [ckptLoadPath, setCkptLoadPath] = useState<string>("");
+  const [probeText, setProbeText] = useState<string>("");
   return (
     <header data-testid="top-bar"
             style={{ height: 56, display: "flex", alignItems: "center",
@@ -201,6 +203,18 @@ export function TopBar(p: TopBarProps): JSX.Element {
                        style={{ width: 200 }} />
               </label>
             </div>
+            <label style={{ padding: "6px 12px", display: "flex",
+                            flexDirection: "column", gap: 3, fontSize: 11,
+                            color: "#374151" }}>
+              <span style={{ color: "#6b7280" }}>probe text:</span>
+              <textarea data-testid="train-probe-text"
+                        placeholder="Optional: encode this for inference probe"
+                        value={probeText}
+                        onChange={(e) => setProbeText(e.target.value)}
+                        rows={2}
+                        style={{ width: 280, fontFamily: "monospace",
+                                 fontSize: 11 }} />
+            </label>
             <button data-testid="run-pipeline-train"
                     onClick={() => { setOpen(false);
                                      p.onRunPipeline("train",
@@ -209,7 +223,9 @@ export function TopBar(p: TopBarProps): JSX.Element {
                                          checkpoint_save_path:
                                            ckptSavePath || undefined,
                                          checkpoint_load_path:
-                                           ckptLoadPath || undefined }); }}
+                                           ckptLoadPath || undefined,
+                                         inference_probe_text:
+                                           probeText || undefined }); }}
                     disabled={!!p.trainDisabled}
                     title={p.trainDisabled?.reason ?? ""}
                     style={{ ...menuItem,

vbgui/tests/TopBar.test.tsx

@@ -148,6 +148,29 @@ describe("TopBar", () => {
         }));
     });
 
+  it("H08: train-probe-text textarea forwards inference_probe_text",
+    () => {
+      const onRunPipeline = vi.fn();
+      render(<TopBar {...defaultTopProps({ onRunPipeline })} />);
+      fireEvent.click(screen.getByTestId("run-pipeline-toggle"));
+      fireEvent.change(screen.getByTestId("train-probe-text"),
+        { target: { value: "hello world from probe" } });
+      fireEvent.click(screen.getByTestId("run-pipeline-train"));
+      expect(onRunPipeline).toHaveBeenLastCalledWith("train",
+        expect.objectContaining({
+          inference_probe_text: "hello world from probe",
+        }));
+    });
+
+  it("H08: empty probe text omits field", () => {
+    const onRunPipeline = vi.fn();
+    render(<TopBar {...defaultTopProps({ onRunPipeline })} />);
+    fireEvent.click(screen.getByTestId("run-pipeline-toggle"));
+    fireEvent.click(screen.getByTestId("run-pipeline-train"));
+    const opts = onRunPipeline.mock.calls.at(-1)?.[1];
+    expect(opts.inference_probe_text).toBeUndefined();
+  });
+
   it("H05: empty checkpoint inputs omit fields", () => {
     const onRunPipeline = vi.fn();
     render(<TopBar {...defaultTopProps({ onRunPipeline })} />);