Add Bell test implementation for quantum entanglement demonstration

[Copilot]

Implements Bell's inequality (CHSH) test to demonstrate quantum entanglement and violation of local realism.

Implementation

• Complex numbers (Complex.ts): Arithmetic operations for quantum state amplitudes
• Quantum states (QuantumState.ts): Two-qubit representation with normalization, Bell state factory for all four maximally entangled states (|Φ±⟩, |Ψ±⟩)
• Bell test (BellTest.ts): Quantum correlation calculation E(a,b) = cos(a-b), CHSH inequality S = |E(a,b) - E(a,b') + E(a',b) + E(a',b')|, optimal measurement angles for maximum violation

Usage

import { BellTest, QuantumState } from './src';

const bellState = QuantumState.createBellState('phi+');
const angles = BellTest.getOptimalCHSHAngles(); // [0, π/2, π/4, 3π/4]
const chshValue = BellTest.calculateCHSH(bellState, angles);

console.log(chshValue); // 2.828 > 2 (classical bound)

Results

• All Bell states achieve S = 2√2 ≈ 2.828, violating classical bound of S ≤ 2
• Product states correctly show S ≈ 0 (no violation)
• 38 tests covering complex arithmetic, state normalization, and correlation calculations

Documentation

• BELLTEST.md: Bell's theorem, CHSH formulation, optimal angles
• demo.ts: Interactive demonstration with theoretical and empirical measurements

Original prompt

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This section details on the original issue you should resolve

<issue_title>Lacking of Bell test.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Lacking of Bell test. #13

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