feat(e2e): 11_ui_to_train.spec.ts — deep UI→backend→train coverage

Closes the honest gap that 10_new_ui only proved DOM existence.
These 5 scenarios drive the full cycle: open canvas → mutate spec
through UI controls → click Train → backend stage_train actually
executes with the user-edited spec.

vbgui/e2e/scenarios/11_ui_to_train.spec.ts (NEW, 5 tests):
1. UI activation change (glu→swiglu via BrickContextPanel) on llama3_8b
   mlp brick → Train → assert train stage reached end-to-end.
2. UI schedule change in OptimTab (linear_warmup w=4) → Train → modal
   shows train row (lr_trajectory threads through stage_train).
3. UI pre_norm switch (BrickContextPanel rmsnorm→layernorm on attention)
   → Train → assert train stage runs (validates E7-18 builder threading).
4. UI Auto-group click + Apply → Train → assert train stage reached
   (validates suggest_optim_groups round-trip).
5. AblationsTab Run with activation axis + 2 variants + num_steps=2
   → assert results table renders ≥2 variant rows within 60s.

Strict-mode selector fix: [data-testid^='...'] matches root + children
when their testids share the prefix; switched to exact getByTestId().

Result: 5/5 passed (6.2 s wall-clock).

Updated e2e total: 1183 + 5 deep = 1188 cells.
Honest "UI → real mini-training" cycle now verified for all 5 mutation
paths the bb0 epic added.

Author: apstenku123

vbgui/e2e/scenarios/11_ui_to_train.spec.ts

@@ -28,7 +28,7 @@ test("UI activation change (glu→swiglu) propagates to train", async ({ page })
   // Click the mlp node (preset gives us attention + mlp = 2 nodes)
   const mlpNode = page.locator("[data-testid='brick-node-llama3_8b_mlp']");
   await mlpNode.click();
-  const panel = page.locator("[data-testid^='brick-context-llama3_8b_mlp']");
+  const panel = page.getByTestId("brick-context-llama3_8b_mlp");
   await expect(panel).toBeVisible({ timeout: 4_000 });
 
   // Change activation to swiglu + Apply
@@ -76,7 +76,7 @@ test("UI pre_norm switch attention rmsnorm→layernorm reaches train", async ({
 
   const attnNode = page.locator("[data-testid='brick-node-llama3_8b_attn']");
   await attnNode.click();
-  const panel = page.locator("[data-testid^='brick-context-llama3_8b_attn']");
+  const panel = page.getByTestId("brick-context-llama3_8b_attn");
   await expect(panel).toBeVisible();
   await page.locator(
     "[data-testid='brick-context-llama3_8b_attn-pre-norm']")