feat(v4-8): RewritersTab → extras.model_summary.rewriters_applied

Closes V4-8 / cppmega-mlx-ais. Closes G7 from V4 audit.

Backend (_summarize_model):
  - model_summary.rewriters_applied = list of rewriter names from
    ctx.spec.rewriters. Surfaces what UI chained so e2e can assert.

UI fix (App.tsx):
  - Real bug: onRewriterApply was never wired, so the conditional
    'Apply chain' button (data-testid=rewriter-apply) never rendered
    even though RewritersTab supports it. Tests had no way to trigger
    a re-verify after chain mutation. Now wired to scheduleVerify().

E2E 22_rewriter_propagation.spec.ts: 3 parametrised scenarios for
MTPRewriter / IFIMRewriter / MHCRewriter. Each adds the rewriter,
verifies the chip appears, clicks Apply, runs Train, asserts the
rewriter name appears in extras.model_summary.rewriters_applied.

3/3 green.

Author: apstenku123

cppmega_v4/runner/stages.py

@@ -696,6 +696,10 @@ def _pget(node: Any, key: str, default: Any) -> Any:
     loss = getattr(spec, "loss", None)
     if loss is not None:
         loss_kind = getattr(loss, "kind", "cross_entropy")
+    # V4-8: surface which rewriters the UI chained so e2e can assert
+    # MTP/IFIM/MHC selection actually reached the backend spec.
+    rewriters = getattr(spec, "rewriters", []) or []
+    rewriters_applied = [getattr(r, "name", str(r)) for r in rewriters]
     return {
         "mlp_activation": _pget(mlp_node, "activation", "swiglu"),
         "attention_pre_norm": _pget(attn_node, "pre_norm", "none"),
@@ -705,6 +709,7 @@ def _pget(node: Any, key: str, default: Any) -> Any:
         "optimizer_kind": optimizer_kind,
         "schedule_kind": schedule_kind,
         "loss_kind": loss_kind,
+        "rewriters_applied": rewriters_applied,
         "num_brick_kinds": len({n.kind for n in nodes}),
     }
 

vbgui/e2e/scenarios/22_rewriter_propagation.spec.ts

@@ -0,0 +1,46 @@
+// V4-8: RewritersTab chain reaches stage_train via spec.rewriters →
+// extras.model_summary.rewriters_applied. Closes G7 from V4 audit.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, closeModal } from "../fixtures";
+import { readTrainExtras } from "../utils/train_extras";
+
+const REWRITERS = ["MTPRewriter", "IFIMRewriter", "MHCRewriter"];
+
+for (const rw of REWRITERS) {
+  test(`V4-8: rewriter '${rw}' propagates to extras.model_summary`,
+    async ({ page }) => {
+      test.setTimeout(60_000);
+      await gotoApp(page);
+      await selectPreset(page, "llama3_8b");
+
+      await page.getByTestId("sidebar-tab-rewriters").click();
+      await page.getByTestId("rewriters-tab").waitFor();
+      await page.getByTestId(`rewriter-add-${rw}`).click();
+      // Chip should appear
+      await expect(page.getByTestId("rewriter-chip-0")).toContainText(rw);
+      await page.getByTestId("rewriter-apply").click();
+
+      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 });
+      const extras = await readTrainExtras(page);
+
+      // Top-level extras has the list (array DOM testid pattern)
+      const rwListRoot = page.locator(
+        "[data-testid^='run-result-extras-train-model_summary-rewriters_applied']");
+      const count = await rwListRoot.count();
+      // model_summary.rewriters_applied renders as nested object → dl entry
+      // OR as array → ol. Either way the chosen rewriter name must appear.
+      // Easier: assert top-level extras renders the array via the recursive
+      // ExtrasEntry pattern (model_summary is an object → values rendered
+      // as JSON.stringify when nested). Check via JSON parse of cell text.
+      const msEntry = await page.getByTestId(
+        "run-result-extras-train-model_summary-rewriters_applied").textContent();
+      expect(msEntry).toContain(rw);
+      expect(extras.losses.every(l => Number.isFinite(l))).toBe(true);
+
+      await closeModal(page);
+    });
+}

vbgui/src/App.tsx

@@ -400,6 +400,7 @@ export function App(): JSX.Element {
                   dispatch({ type: "rewriters.remove", index: i })}
                 onRewriterReorder={(f, t) =>
                   dispatch({ type: "rewriters.reorder", from: f, to: t })}
+                onRewriterApply={() => void scheduleVerify()}
                 onShardingChange={(s) =>
                   dispatch({ type: "sharding.set", sharding: s })}
                 onShardingAccept={handleShardingAccept}