test(v4-5): all 11 ActivationName values through UI→train

Closes V4-5 / cppmega-mlx-doa. V3-5 only proved swiglu propagation;
v4-5 parametrises across the full 11-entry ACTIVATION_OPTIONS set
declared in BrickContextPanel.tsx:

  glu, gelu, relu, relu2, sqrelu, silu, mish, swiglu, geglu, reglu, xielu

Each cell: click llama3_8b_mlp brick → BrickContextPanel → set
activation → Apply → Train → assert extras.model_summary.mlp_activation
matches selection AND losses finite AND weight_delta>0.

11/11 green in 14.4s. Backend support pre-verified via direct
BLOCK_BUILDERS['mlp'] smoke for every activation.

Author: apstenku123

vbgui/e2e/scenarios/19_activation_propagation.spec.ts

@@ -0,0 +1,42 @@
+// V4-5: Every ActivationName must propagate UI → BrickContextPanel →
+// stage_train → extras.model_summary.mlp_activation. V3-5 proved only
+// swiglu; this proves all 11.
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, selectPreset, clickRunPipeline, closeModal } from "../fixtures";
+import { readTrainExtras } from "../utils/train_extras";
+
+// Order matches ACTIVATION_OPTIONS in BrickContextPanel.tsx.
+const ACTIVATIONS = [
+  "glu", "gelu", "relu", "relu2", "sqrelu", "silu", "mish",
+  "swiglu", "geglu", "reglu", "xielu",
+];
+
+for (const act of ACTIVATIONS) {
+  test(`V4-5: UI activation '${act}' reaches extras.model_summary`,
+    async ({ page }) => {
+      test.setTimeout(60_000);
+      await gotoApp(page);
+      await selectPreset(page, "llama3_8b");
+
+      await page.locator("[data-testid='brick-node-llama3_8b_mlp']").click();
+      const panel = page.getByTestId("brick-context-llama3_8b_mlp");
+      await expect(panel).toBeVisible({ timeout: 4_000 });
+      await page.locator(
+        "[data-testid='brick-context-llama3_8b_mlp-activation']")
+        .selectOption(act);
+      await page.locator(
+        "[data-testid='brick-context-llama3_8b_mlp-apply']").click();
+
+      await clickRunPipeline(page, "train");
+      const extras = await readTrainExtras(page);
+
+      // Propagation assertion
+      expect(extras.model_summary.mlp_activation).toBe(act);
+      // Math sanity
+      expect(extras.losses.every(l => Number.isFinite(l))).toBe(true);
+      expect(extras.weight_delta_norm).toBeGreaterThan(0);
+
+      await closeModal(page);
+    });
+}