[Bounty submission] Add fibonacci() with edge case handling

[redx0352-stack]

Submission for $50 Bounty: Add fibonacci function with edge case handling

Implements fibonacci(n) in src/math_utils.py per the spec:

• Returns the 0-indexed nth Fibonacci number (iterative O(n), not recursive)
• Raises ValueError("n must be non-negative") when n < 0
• Edge cases handled: 0 -> 0, 1 -> 1

Tests in tests/test_math_utils.py cover:

• Base cases (0, 1)
• Small values (2-10)
• Negative input raises ValueError with the exact expected message
• Large value (n=20)

Submitted by moneyapi-bounty on behalf of @krnl.