feat(v7-i03): synchronous trainInFlight lock closes H22 race window

Closes V7-I03 (cppmega-mlx-pi1a): H22 disabled the Train button via
the trainInFlight React state, but React commits state updates
asynchronously so a 0ms-delay double click could pass the
prop-disabled check on both invocations and fire pipeline.run
twice.

Fix: trainInFlightLockRef (useRef<boolean>) mutated SYNCHRONOUSLY
at the top of handleRunPipeline before any await. The second
microtask click reads the ref already true and returns early.
Released in the finally block so legitimate sequential trains
work normally.

Tests:
- vbgui e2e 79_concurrent_train_race.spec.ts: zero-delay double
  click via page.evaluate(() => { btn.click(); btn.click(); })
  → asserts exactly 1 pipeline.run HTTP POST. 1/1 passing.
- vbgui e2e concurrency regression (H03 + H22 + V7-I03): 3/3
  passing — no regression in cancel or button-disabled flows.

Author: apstenku123

vbgui/e2e/scenarios/79_concurrent_train_race.spec.ts

@@ -0,0 +1,42 @@
+// V7-I03: tightens H22 with a synchronous lock. Two rapid Train
+// clicks dispatched in the same microtask (no artificial delay)
+// must result in only ONE pipeline.run on the backend.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+
+test("V7-I03: zero-delay double Train click → single pipeline.run",
+  async ({ page }) => {
+    test.setTimeout(60_000);
+
+    // Count pipeline.run HTTP requests across the test.
+    let pipelineRunCount = 0;
+    page.on("request", (req) => {
+      if (!req.url().endsWith("/rpc") || req.method() !== "POST") return;
+      try {
+        const body = JSON.parse(req.postData() ?? "{}");
+        if (body.method === "pipeline.run") pipelineRunCount += 1;
+      } catch { /* ignore */ }
+    });
+
+    await gotoApp(page);
+    await selectPreset(page, "llama3_8b");
+    await page.getByTestId("run-pipeline-toggle").click();
+    await page.getByTestId("train-num-steps").fill("2");
+    // Dispatch two clicks in the SAME microtask via JS so neither
+    // sees a React render before the second fires.
+    await page.evaluate(() => {
+      const btn = document.querySelector<HTMLButtonElement>(
+        "[data-testid='run-pipeline-train']")!;
+      btn.click();
+      btn.click();
+    });
+
+    // Wait for the modal of the (one and only) pipeline run.
+    await page.getByTestId("run-result-modal").waitFor({ timeout: 60_000 });
+    await closeModal(page);
+
+    // Allow a moment for any straggler request to land.
+    await page.waitForTimeout(200);
+    expect(pipelineRunCount).toBe(1);
+  });

vbgui/src/App.tsx

@@ -107,6 +107,12 @@ export function App(): JSX.Element {
   const [runReport, setRunReport] = useState<RunReport | null>(null);
   const [runError, setRunError] = useState<string | null>(null);
   const [trainInFlight, setTrainInFlight] = useState(false);
+  // V7-I03: synchronous lock. React's setTrainInFlight schedules an
+  // async commit, so two button clicks within the same microtask
+  // both read trainInFlight=false and both call rpc.call. The ref
+  // is mutated synchronously inside handleRunPipeline before any
+  // await, closing the ~10ms window.
+  const trainInFlightLockRef = useRef<boolean>(false);
   const [trainRunId, setTrainRunId] = useState<string | null>(null);
   // H04: most recent successfully-completed Train run_id, used as
   // continue_from_run_id when the warm-start checkbox is on. Cleared
@@ -371,8 +377,17 @@ export function App(): JSX.Element {
       master_dtype?: "fp32" | "bf16" | "fp16" | "auto";
     },
   ) => {
+    // V7-I03: synchronous lock check at the very top — before any
+    // await, before the canvas-empty guard. Two rapid clicks both
+    // pass through React's stale-prop trainInFlight=false but only
+    // the first acquires the ref.
+    if (mode === "train") {
+      if (trainInFlightLockRef.current) return;
+      trainInFlightLockRef.current = true;
+    }
     const snap = wireSpecRef.current;
     if (snap.nodes.length === 0) {
+      if (mode === "train") trainInFlightLockRef.current = false;
       setRunError("canvas is empty — drop bricks or pick a preset first");
       setRunReport(null);
       return;
@@ -471,6 +486,7 @@ export function App(): JSX.Element {
       if (mode === "train") {
         setTrainInFlight(false);
         setTrainRunId(null);
+        trainInFlightLockRef.current = false;
       }
     }
   }, [rpc, trainParquetPath, trainSideChannels, trainTokenizerPath,