feat(v7-g05): FIM toggle in TopBar threads to stage_train.fim_enabled

UI gap: backend supports opts.fim_enabled and surfaces extras.fim_active
+ fim_ratio (stages.py:1371,1694-95) since V6, but no TopBar control
exposed it. Honest-closure 'FIM trajectory through UI' was reporting
no UI affordance — this wires it end-to-end.

- TopBar: new train-fim-enabled checkbox inside run-pipeline menu.
- App.tsx: forwards opts.fim_enabled into stage_options.train.
- e2e 80: two scenarios — toggle on asserts fim_active=true plus
  fim_ratio in (0,1] via the StageExtras DOM render; toggle off
  asserts fim_active=false. Real pipeline.run, not mocked.

Author: apstenku123

vbgui/e2e/scenarios/80_fim_toggle.spec.ts

@@ -0,0 +1,59 @@
+// V7-G05: FIM (Fill-In-Middle) toggle in TopBar threads through
+// stage_options.train.fim_enabled and surfaces extras.train.fim_active
+// + fim_ratio. The visible-row assertion uses the StageExtras render
+// in RunResultModal so this is a true "UI shows backend math" check,
+// not just RPC payload echo.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+
+test("V7-G05: FIM toggle surfaces fim_active=true + fim_ratio in train extras",
+  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-num-steps").fill("2");
+    await page.getByTestId("train-fim-enabled").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 fimActive = await page
+      .getByTestId("run-result-extras-train-fim_active").textContent();
+    expect(fimActive).toBe("true");
+
+    const fimRatioText = await page
+      .getByTestId("run-result-extras-train-fim_ratio").textContent();
+    expect(fimRatioText).not.toBe("null");
+    const fimRatio = parseFloat(fimRatioText ?? "0");
+    expect(fimRatio).toBeGreaterThan(0);
+    expect(fimRatio).toBeLessThanOrEqual(1);
+
+    await closeModal(page);
+  });
+
+test("V7-G05: FIM off by default — fim_active=false",
+  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-num-steps").fill("2");
+    // Do not check train-fim-enabled.
+    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 fimActive = await page
+      .getByTestId("run-result-extras-train-fim_active").textContent();
+    expect(fimActive).toBe("false");
+
+    await closeModal(page);
+  });

vbgui/src/App.tsx

@@ -377,6 +377,7 @@ export function App(): JSX.Element {
       checkpoint_load_path?: string;
       inference_probe_text?: string;
       master_dtype?: "fp32" | "bf16" | "fp16" | "auto";
+      fim_enabled?: boolean;
     },
   ) => {
     // V7-I03: synchronous lock check at the very top — before any
@@ -433,6 +434,11 @@ export function App(): JSX.Element {
       if (opts?.master_dtype && opts.master_dtype !== "auto") {
         trainOpts.master_dtype = opts.master_dtype;
       }
+      // V7-G05: forward FIM toggle. stage_train opts.fim_enabled gates
+      // the middle-third masking + fim_ratio extras.
+      if (opts?.fim_enabled) {
+        trainOpts.fim_enabled = true;
+      }
       // H20: when the spec carries sharding axes, derive fake_ranks
       // from the product of their degrees so a Train run simulates a
       // mean-reduced multi-rank backward (extras.fake_ranks +

vbgui/src/components/TopBar.tsx

@@ -19,6 +19,7 @@ export interface TopBarProps {
       checkpoint_load_path?: string;
       inference_probe_text?: string;
       master_dtype?: "fp32" | "bf16" | "fp16" | "auto";
+      fim_enabled?: boolean;
     }) => void;
   /** H02: toggle callbacks. */
   onMixedPrecisionChange?: (enabled: boolean) => void;
@@ -58,6 +59,10 @@ export function TopBar(p: TopBarProps): JSX.Element {
   // fp32/bf16/fp16 options override that for H23.
   const [masterDtype, setMasterDtype] =
     useState<"fp32" | "bf16" | "fp16" | "auto">("auto");
+  // V7-G05: FIM (Fill-In-Middle) data path toggle. When on, stage_train
+  // surfaces extras.train.fim_active + fim_ratio so the UI honest-closure
+  // shows the FIM math actually fired.
+  const [fimEnabled, setFimEnabled] = useState<boolean>(false);
   return (
     <header data-testid="top-bar"
             style={{ height: 56, display: "flex", alignItems: "center",
@@ -247,6 +252,14 @@ export function TopBar(p: TopBarProps): JSX.Element {
                 <option value="fp16">fp16</option>
               </select>
             </label>
+            <label style={{ padding: "6px 12px", display: "flex",
+                            alignItems: "center", gap: 6, fontSize: 11,
+                            color: "#374151" }}>
+              <input data-testid="train-fim-enabled" type="checkbox"
+                     checked={fimEnabled}
+                     onChange={(e) => setFimEnabled(e.target.checked)} />
+              FIM (Fill-In-Middle) data path
+            </label>
             <label style={{ padding: "6px 12px", display: "flex",
                             flexDirection: "column", gap: 3, fontSize: 11,
                             color: "#374151" }}>
@@ -270,7 +283,8 @@ export function TopBar(p: TopBarProps): JSX.Element {
                                            ckptLoadPath || undefined,
                                          inference_probe_text:
                                            probeText || undefined,
-                                         master_dtype: masterDtype }); }}
+                                         master_dtype: masterDtype,
+                                         fim_enabled: fimEnabled }); }}
                     // H22: disable while a Train is already running so
                     // double-clicks don't spawn a parallel pipeline.
                     disabled={!!p.trainDisabled || !!p.trainInFlight}