test(e2e): add --parallel flag execution test

Package b depends on a, so without --parallel they run sequentially.
Both use a barrier requiring 2 participants — completing without
timeout proves --parallel discards edges and runs them concurrently.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "parallel-execution-test",
+  "private": true
+}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/packages/a/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@parallel/a",
+  "scripts": {
+    "build": "vtt barrier ../../.barrier sync 2"
+  }
+}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/packages/b/package.json

@@ -0,0 +1,9 @@
+{
+  "name": "@parallel/b",
+  "scripts": {
+    "build": "vtt barrier ../../.barrier sync 2"
+  },
+  "dependencies": {
+    "@parallel/a": "workspace:*"
+  }
+}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/pnpm-workspace.yaml

@@ -0,0 +1,2 @@
+packages:
+  - packages/*

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/snapshots.toml

@@ -0,0 +1,8 @@
+# Package b depends on a, so without --parallel they run sequentially.
+# Both use a barrier requiring 2 participants — if run sequentially the
+# first would wait forever and the test would timeout.
+# --parallel discards dependency edges, allowing both to run at once.
+
+[[e2e]]
+name = "parallel flag runs dependent tasks concurrently"
+steps = [["vt", "run", "-r", "--parallel", "build"]]

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/snapshots/parallel flag runs dependent tasks concurrently.snap

@@ -0,0 +1,11 @@
+---
+source: crates/vite_task_bin/tests/e2e_snapshots/main.rs
+expression: e2e_outputs
+---
+> vt run -r --parallel build
+~/packages/a$ vtt barrier ../../.barrier sync 2 ⊘ cache disabled
+~/packages/b$ vtt barrier ../../.barrier sync 2 ⊘ cache disabled
+
+
+---
+vt run: 0/2 cache hit (0%). (Run `vt run --last-details` for full details)

crates/vite_task_bin/tests/e2e_snapshots/fixtures/parallel-execution/vite-task.json

@@ -0,0 +1,3 @@
+{
+  "cache": false
+}