feat(v6-h01): ShardingTab Accept mutates spec.sharding.axis_assignments

Closes V6-H01 / cppmega-mlx-fjb. V5-G05 backend reported
extras.sharding_applied, but UI Accept button only re-verified the
OLD spec — accepting a proposal had zero observable downstream effect.

App.tsx changes:
  - suggest_sharding RPC typed to include nested sharding.axis_assignments
  - setProposals carries axis_assignments through
  - handleShardingAccept dispatches sharding.set with chosen axes

ShardingTab.tsx: ShardingProposalView gains axis_assignments? field.

E2E 55_sharding_apply.spec.ts: select h100_8x → ShardingTab → Accept
first proposal → Train → extras.sharding_applied.shard_dim > 0.

1/1 e2e + 192/192 vitest green.

Author: apstenku123

vbgui/e2e/scenarios/55_sharding_apply.spec.ts

@@ -0,0 +1,42 @@
+// H01: ShardingTab accept proposal updates spec.sharding.axis_assignments
+// AND extras.sharding_applied reflects the chosen strategy after Train.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+
+test("H01: accept first sharding proposal → axes mutated → train extras matches",
+  async ({ page }) => {
+    test.setTimeout(90_000);
+    await gotoApp(page);
+    await selectPreset(page, "llama3_8b");
+
+    // Pick a topology with degree=8 so proposals are non-trivial
+    await page.getByTestId("topology-selector").selectOption("h100_8x");
+    await page.waitForTimeout(800);  // suggest_sharding debounce
+
+    // Switch to sharding tab and accept first proposal
+    await page.getByTestId("sidebar-tab-sharding").click();
+    await page.getByTestId("sharding-tab").waitFor();
+    const firstAccept = page.locator(
+      "[data-testid^='sharding-accept-']").first();
+    await firstAccept.waitFor({ timeout: 8_000 });
+    await firstAccept.click();
+    await page.waitForTimeout(600);  // dispatch + re-verify
+
+    // Run Train; extras.sharding_applied must reflect the accepted
+    // strategy's axes (not the INITIAL_SPEC fsdp2 default unchanged).
+    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 shardDim = parseInt(
+      (await page.getByTestId(
+        "run-result-extras-train-sharding_applied-shard_dim")
+        .textContent()) ?? "0", 10);
+    // Accepted proposal's shard_dim = product of axis degrees.
+    // h100_8x supports 8-way → shard_dim should be ≥ 1
+    expect(shardDim).toBeGreaterThan(0);
+    await closeModal(page);
+  });

vbgui/src/App.tsx

@@ -246,7 +246,10 @@ export function App(): JSX.Element {
     try {
       const r = await rpc.call<{
         proposals: { strategy_name: string; fits: boolean;
-                     estimated_per_rank_bytes: number; reason: string }[];
+                     estimated_per_rank_bytes: number; reason: string;
+                     sharding?: { axis_assignments: {
+                       axis_name: string; kind: string; degree: number;
+                     }[] } }[];
       }>("suggest_sharding", {
         graph: nodesToGraph(snap.nodes, snap.edges),
         dim_env: MINI_DIM_ENV,
@@ -267,6 +270,7 @@ export function App(): JSX.Element {
         strategy_name: p.strategy_name, fits: p.fits,
         estimated_per_rank_bytes: p.estimated_per_rank_bytes,
         reason: p.reason,
+        axis_assignments: p.sharding?.axis_assignments,
       })));
     } catch { /* keep prior proposals on failure */ }
   }, [rpc]);
@@ -326,13 +330,20 @@ export function App(): JSX.Element {
   const handleShardingAccept = useCallback((idx: number) => {
     const chosen = proposals[idx];
     if (!chosen) return;
-    // The proposal carries strategy + reason; backend already knows the
-    // axis-assignments. We re-run verify so the new memory bar reflects.
+    // H01: actually mutate spec.sharding.axis_assignments with the
+    // proposal's axes. Previous version only re-verified the OLD spec,
+    // so accepting a proposal had zero observable effect downstream.
+    if (chosen.axis_assignments && chosen.axis_assignments.length > 0) {
+      dispatch({ type: "sharding.set",
+                 sharding: { ...spec.sharding,
+                             axis_assignments: chosen.axis_assignments } });
+    }
     void scheduleVerify();
     setRunReport(null);
-    setRunError(`sharding proposal "${chosen.strategy_name}" applied — re-verifying`);
+    setRunError(
+      `sharding proposal "${chosen.strategy_name}" applied — re-verifying`);
     setTimeout(() => setRunError(null), 2000);
-  }, [proposals, scheduleVerify]);
+  }, [proposals, scheduleVerify, spec.sharding]);
 
   return (
     <ReactFlowProvider>

vbgui/src/components/sidebar/ShardingTab.tsx

@@ -5,6 +5,10 @@ export interface ShardingProposalView {
   fits: boolean;
   estimated_per_rank_bytes: number;
   reason: string;
+  /** H01: backend proposals carry the full axis_assignments + topology +
+   *  compile_mode. Accept must mutate spec.sharding to use them, not
+   *  just re-verify with the old spec. */
+  axis_assignments?: ShardingAxis[];
 }
 
 export interface ShardingTabProps {