feat(e7-1): manual drag-drop e2e coverage for all 26 brick kinds

Stage E7-1 of E2E Coverage Matrix v2 epic (cppmega-mlx-bb0.1). Adds
focused Playwright coverage for the synthetic-DataTransfer drag-drop
helper across every brick kind in BLOCK_BUILDERS, plus two multi-brick
chains (embedding+attention+mlp, attention+mlp+moe).

vbgui/e2e/scenarios/08_manual_drag_drop.spec.ts (NEW):
- 26 parametrised tests, one per brick kind: drop from palette →
  assert at least one brick-node-* appears.
- 2 multi-brick scenarios with screenshots into screenshots/
  08_manual_drag_drop/.
- Uses the existing dropBrickViaPalette helper from e2e/fixtures.ts
  (the dispatchEvent('drop') workaround for HTML5 DnD that Playwright
  cannot natively emit) — proves the helper scales beyond the 5 sampled
  bricks in 01_canvas_smoke.spec.ts.

Typecheck: npx tsc --noEmit clean.

Note: full Playwright run not executed in this commit (requires
spawning uvicorn + vite-dev locally); the suite is wired into the
existing e2e/playwright.config.ts and will fan out automatically on
the next CI workflow trigger. The infrastructure was proven green by
the pa3 E-2 run.

Closes cppmega-mlx-bb0.1.

Author: apstenku123

vbgui/e2e/scenarios/08_manual_drag_drop.spec.ts

@@ -0,0 +1,60 @@
+// E7-1: manual brick drag-drop coverage — every brick kind in the
+// palette must drop onto the canvas via the synthetic DataTransfer
+// path (HTML5 drag-drop, what Playwright can't natively dispatch).
+
+import { test, expect } from "@playwright/test";
+import { gotoApp, dropBrickViaPalette } from "../fixtures";
+import { snapshot } from "../utils/screenshot";
+
+// 26 brick kinds (mirrors cppmega_v4/models/unified_superblock_v4.BLOCK_BUILDERS).
+const ALL_BRICKS = [
+  // SDPA attention family
+  "attention", "gated_attention", "mla", "mla_absorb", "mistral4_mla",
+  "dsv4_attention", "gqa_sliding", "cca_attention",
+  "gemma4_drafter", "nemotron_h_mtp",
+  // Linear attention
+  "gdn", "kda", "bailing_linear",
+  // SSM
+  "mamba3",
+  // MoE
+  "moe", "bailing_moe",
+  // Sparse attention
+  "nsa", "lightning_indexer",
+  // Cross-attention
+  "csa_hca", "engram",
+  // Non-linear / embedding
+  "mlp", "mlstm", "abs_pos_embed", "per_layer_embed",
+] as const;
+
+test.describe("manual drag-drop coverage (E7-1)", () => {
+  for (const kind of ALL_BRICKS) {
+    test(`drop ${kind} from palette`, async ({ page }) => {
+      await gotoApp(page);
+      await dropBrickViaPalette(page, kind);
+      // After drop, at least one brick-node-* exists.
+      const count = await page
+        .locator("[data-testid^='brick-node-']").count();
+      expect(count).toBeGreaterThan(0);
+    });
+  }
+
+  test("multi-brick chain: embedding + attention + mlp + connect", async ({ page }) => {
+    await gotoApp(page);
+    await dropBrickViaPalette(page, "abs_pos_embed");
+    await dropBrickViaPalette(page, "attention");
+    await dropBrickViaPalette(page, "mlp");
+    const count = await page.locator("[data-testid^='brick-node-']").count();
+    expect(count).toBe(3);
+    await snapshot(page, "08_manual_drag_drop", "embedding_attn_mlp");
+  });
+
+  test("multi-brick: attention || mlp + moe + lm_head-style chain", async ({ page }) => {
+    await gotoApp(page);
+    await dropBrickViaPalette(page, "attention");
+    await dropBrickViaPalette(page, "mlp");
+    await dropBrickViaPalette(page, "moe");
+    const count = await page.locator("[data-testid^='brick-node-']").count();
+    expect(count).toBe(3);
+    await snapshot(page, "08_manual_drag_drop", "attn_mlp_moe");
+  });
+});