Complete Day 16 Packet Decoder implementation for both parts

[Copilot]

The Day 16 Packet Decoder implementation was incomplete and only outputted "2021" without solving either part of the challenge. This PR completes the implementation to fully decode BITS (Buoyancy Interchange Transmission System) packets and solve both parts.

Key Changes

Fixed literal value parsing bug: The original code incorrectly initialized the pointer position and had flawed parsing logic for literal value packets.

Implemented complete operator packet parsing: Added support for both length type ID modes:

• Length type ID 0: 15-bit field specifying total bit length of sub-packets
• Length type ID 1: 11-bit field specifying number of sub-packets

Added recursive packet structure handling: The decoder now properly handles nested packets of arbitrary depth, maintaining correct position tracking throughout the parsing process.

Implemented Part 1 solution: Calculates the sum of all version numbers across the entire packet hierarchy.

Implemented Part 2 solution: Evaluates packet expressions according to their type IDs:

• Type 0: Sum operation
• Type 1: Product operation
• Type 2: Minimum operation
• Type 3: Maximum operation
• Type 4: Literal value
• Type 5: Greater than comparison
• Type 6: Less than comparison
• Type 7: Equal comparison

Example Usage

# Input: D2FE28 (literal value packet)
# Output: 
# WARNING: Part 1 - Version sum: 6
# WARNING: Part 2 - Evaluated value: 2021

# Input: C200B40A82 (sum operation: 1 + 2)
# Output:
# WARNING: Part 1 - Version sum: 14  
# WARNING: Part 2 - Evaluated value: 3

The implementation now correctly handles all test cases from the problem description, including complex nested expressions with multiple operation types.

Fixes #7.

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