feat(h04): train-warm-start checkbox forwards continue_from_run_id

TopBar gains `train-warm-start` checkbox in the Train dropdown menu and
threads a `warm_start: boolean` flag into `onRunPipeline("train", ...)`.
App tracks `lastTrainRunId` (set after a Train stage completes with
status="ok") and, when warm_start is on, includes
`continue_from_run_id=lastTrainRunId` in stage_options.train.

Backend V5-G10 (stages.py:770) already restored opt.state from the
_RUN_CACHE on hit and emits extras.opt_state_carried=true. v6 closes
the UI side so a user can actually exercise warm-start without editing
JSON.

Tests:
- vbgui vitest TopBar.test.tsx: train-warm-start checkbox flips
  warm_start flag in onRunPipeline payload (default OFF → false,
  toggled ON → true) — 18/18 TopBar tests passing.
- vbgui vitest full suite: 198/198 passing.
- vbgui e2e 58_warm_start.spec.ts: two consecutive Train runs from the
  UI — first OFF asserts extras.opt_state_carried="false", second ON
  asserts extras.opt_state_carried="true".
- vbgui e2e V6 suite (55+56+57+58): 5/5 passing — no regression.
- backend pytest stage_train+warm: 40/40 passing.

Author: apstenku123

vbgui/e2e/scenarios/58_warm_start.spec.ts

@@ -0,0 +1,46 @@
+// H04: train-warm-start checkbox forwards continue_from_run_id so the
+// backend G10 LRU cache restores opt.state for the next Train run.
+//
+// First Train (warm-start OFF):  cold opt.state → extras.opt_state_carried=false
+// Second Train (warm-start ON):  same spec re-runs → opt_state_carried=true.
+//
+// The second run does not require a strictly lower losses[0] (the cached
+// opt.state pairs with a freshly built model whose weights re-randomise,
+// so the loss surface differs). What MUST hold is that the warm-start
+// flag actually flips the backend's opt_state_carried extras.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+import { readTrainExtras } from "../utils/train_extras";
+
+test("H04: warm-start checkbox flips extras.opt_state_carried",
+  async ({ page }) => {
+    test.setTimeout(120_000);
+    await gotoApp(page);
+    await selectPreset(page, "llama3_8b");
+
+    // First Train — warm-start OFF (default). Expect opt_state_carried=false.
+    await page.getByTestId("run-pipeline-toggle").click();
+    await page.getByTestId("train-num-steps").fill("2");
+    // Confirm the checkbox starts unchecked.
+    await expect(page.getByTestId("train-warm-start")).not.toBeChecked();
+    await page.getByTestId("run-pipeline-train").click();
+    await page.getByTestId("run-result-modal").waitFor({ timeout: 60_000 });
+    const firstExtras = await readTrainExtras(page);
+    expect(firstExtras.losses.length).toBe(2);
+    const firstCarried = await page.getByTestId(
+      "run-result-extras-train-opt_state_carried").textContent();
+    expect(firstCarried?.trim().toLowerCase()).toBe("false");
+    await closeModal(page);
+
+    // Second Train — warm-start ON. Expect opt_state_carried=true.
+    await page.getByTestId("run-pipeline-toggle").click();
+    await page.getByTestId("train-warm-start").check();
+    await expect(page.getByTestId("train-warm-start")).toBeChecked();
+    await page.getByTestId("run-pipeline-train").click();
+    await page.getByTestId("run-result-modal").waitFor({ timeout: 60_000 });
+    const secondCarried = await page.getByTestId(
+      "run-result-extras-train-opt_state_carried").textContent();
+    expect(secondCarried?.trim().toLowerCase()).toBe("true");
+    await closeModal(page);
+  });

vbgui/src/App.tsx

@@ -94,6 +94,10 @@ export function App(): JSX.Element {
   const [runError, setRunError] = useState<string | null>(null);
   const [trainInFlight, setTrainInFlight] = useState(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
+  // when an error/cancel terminates the next run.
+  const [lastTrainRunId, setLastTrainRunId] = useState<string | null>(null);
   const [selectedBrickId, setSelectedBrickId] = useState<string | null>(null);
   const [inferenceLog, setInferenceLog] = useState<
     { brick: string; param: string; value: unknown;
@@ -289,7 +293,7 @@ export function App(): JSX.Element {
 
   const handleRunPipeline = useCallback(async (
     mode: RunMode,
-    opts?: { num_steps?: number },
+    opts?: { num_steps?: number; warm_start?: boolean },
   ) => {
     const snap = wireSpecRef.current;
     if (snap.nodes.length === 0) {
@@ -313,6 +317,11 @@ export function App(): JSX.Element {
       if (typeof opts?.num_steps === "number") {
         trainOpts.num_steps = opts.num_steps;
       }
+      // H04: warm-start uses lastTrainRunId as continue_from_run_id so
+      // the backend G10 LRU cache restores opt.state from prior run.
+      if (opts?.warm_start && lastTrainRunId) {
+        trainOpts.continue_from_run_id = lastTrainRunId;
+      }
       if (trainParquetPath) trainOpts.parquet_path = trainParquetPath;
       if (trainTokenizerPath) trainOpts.tokenizer_path = trainTokenizerPath;
       // Forward SideChannelsTab train selection as synthetic int lists for the
@@ -336,6 +345,14 @@ export function App(): JSX.Element {
         pipeline: { stages, stage_options },
       });
       setRunReport(r);
+      // H04: remember run_id of a successful Train so a follow-up
+      // warm-start run can reference it.
+      if (mode === "train" && activeTrainRunId) {
+        const trainStage = r.stages?.find((s) => s.name === "train");
+        if (trainStage?.status === "ok") {
+          setLastTrainRunId(activeTrainRunId);
+        }
+      }
     } catch (e) {
       setRunError(String(e));
     } finally {
@@ -344,7 +361,8 @@ export function App(): JSX.Element {
         setTrainRunId(null);
       }
     }
-  }, [rpc, trainParquetPath, trainSideChannels, trainTokenizerPath]);
+  }, [rpc, trainParquetPath, trainSideChannels, trainTokenizerPath,
+      lastTrainRunId]);
 
   const handleCancelTrain = useCallback(async () => {
     const runId = trainRunId;

vbgui/src/components/TopBar.tsx

@@ -14,7 +14,7 @@ export interface TopBarProps {
   onTopologyChange: (t: TopologyFactory) => void;
   onCompileModeChange: (m: SpecState["sharding"]["compile_mode"]) => void;
   onRunPipeline: (mode: RunMode,
-    opts?: { num_steps?: number }) => void;
+    opts?: { num_steps?: number; warm_start?: boolean }) => void;
   /** H02: toggle callbacks. */
   onMixedPrecisionChange?: (enabled: boolean) => void;
   onFp8EnabledChange?: (enabled: boolean) => void;
@@ -39,6 +39,7 @@ export interface TopBarProps {
 export function TopBar(p: TopBarProps): JSX.Element {
   const [open, setOpen] = useState(false);
   const [trainNumSteps, setTrainNumSteps] = useState<number>(2);
+  const [warmStart, setWarmStart] = useState<boolean>(false);
   return (
     <header data-testid="top-bar"
             style={{ height: 56, display: "flex", alignItems: "center",
@@ -165,10 +166,19 @@ export function TopBar(p: TopBarProps): JSX.Element {
                          e.target.value || "1", 10)))}
                      style={{ width: 50 }} />
             </div>
+            <label style={{ padding: "6px 12px", display: "flex",
+                            alignItems: "center", gap: 6, fontSize: 11,
+                            color: "#374151" }}>
+              <input data-testid="train-warm-start" type="checkbox"
+                     checked={warmStart}
+                     onChange={(e) => setWarmStart(e.target.checked)} />
+              warm-start (continue from last run)
+            </label>
             <button data-testid="run-pipeline-train"
                     onClick={() => { setOpen(false);
                                      p.onRunPipeline("train",
-                                       { num_steps: trainNumSteps }); }}
+                                       { num_steps: trainNumSteps,
+                                         warm_start: warmStart }); }}
                     disabled={!!p.trainDisabled}
                     title={p.trainDisabled?.reason ?? ""}
                     style={{ ...menuItem,

vbgui/tests/TopBar.test.tsx

@@ -131,6 +131,22 @@ describe("TopBar", () => {
       expect.objectContaining({ num_steps: expect.any(Number) }));
   });
 
+  it("H04: train-warm-start checkbox forwards warm_start flag", () => {
+    const onRunPipeline = vi.fn();
+    render(<TopBar {...defaultTopProps({ onRunPipeline })} />);
+    fireEvent.click(screen.getByTestId("run-pipeline-toggle"));
+    // Default OFF → warm_start: false
+    fireEvent.click(screen.getByTestId("run-pipeline-train"));
+    expect(onRunPipeline).toHaveBeenLastCalledWith("train",
+      expect.objectContaining({ warm_start: false }));
+    // Toggle ON → warm_start: true
+    fireEvent.click(screen.getByTestId("run-pipeline-toggle"));
+    fireEvent.click(screen.getByTestId("train-warm-start"));
+    fireEvent.click(screen.getByTestId("run-pipeline-train"));
+    expect(onRunPipeline).toHaveBeenLastCalledWith("train",
+      expect.objectContaining({ warm_start: true }));
+  });
+
   it("does not render legacy train side-channel checkboxes", () => {
     render(<TopBar {...defaultTopProps()} />);
     fireEvent.click(screen.getByTestId("run-pipeline-toggle"));