Add fixed-capacity support to Deque with ring buffer semantics

[ekansh28]

Implements a complete fixed-capacity Deque feature (issue #309), providing ring buffer behavior backed by contiguous memory.

Core Features:

• Added Deque<T>(fixedCapacity:) initializer for bounded deques.
• Implemented ring buffer semantics: • append(_:) removes the oldest element when full. • prepend(_:) removes the newest element when full.
• Introduced helper properties: • maxCapacity • isFixedCapacity • isFull • remainingCapacity
• Ensured full backward compatibility — existing unbounded deques continue to function unchanged.

Behavior:

• Example: [1, 2, 3] → append(4) → [2, 3, 4]
• Example: [1, 2, 3] → prepend(0) → [0, 1, 2]

Test Coverage:

• Property validation (capacity, fullness, etc.)
• Append/prepend ring buffer behavior
• Precondition validation
• Integration tests with existing Deque functionality

This change delivers the requested feature for high-performance systems that need bounded memory collections. It follows Swift Collections’ coding and documentation conventions, while maintaining O(1) amortized performance guarantees.

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[ekansh28]

@lorentey