test: run the suite and benchmark with Bun's native test runner

Replace Vitest with bun test across the whole suite. A preload compiles
components through Angular's JIT transform with inlined templates, sets
up jsdom, TestBed and zoneless change detection, and a small shim covers
the Vitest APIs bun:test lacks. Files run isolated, real-worker specs
spawn the running Bun, and line coverage from lcov is converted to the
existing HTML and Cobertura reports. The CI benchmark replaces its heap
cap with a container memory limit. Vitest, Jest, Nx and their Vite
configuration are removed.

Refs #291

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/services/leave-planner-explanation.spec.ts

@@ -1,5 +1,6 @@
 import { createOptimizationPresetRules } from './rule-service';
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it } from 'bun:test';
+import { vi } from '../../testing/vitest-compat';
 import { createPlanningCalculation } from './leave-planner.service';
 
 const balance = (