Security audit and comprehensive testing framework for vector library

[Copilot]

This PR addresses critical security vulnerabilities and adds comprehensive testing to the C dynamic array library. The audit identified several memory safety issues and missing safeguards that could lead to memory leaks, buffer overflows, and undefined behavior in multithreaded environments.

Critical Security Fixes

Memory Leak in vector_pop()

The original vector_pop() function allocated memory for returned elements but didn't document that callers must free this memory, leading to inevitable memory leaks:

// Before: Memory leak waiting to happen
int* value = vector_pop(int, v);  // Who frees this?

// After: Safe alternatives provided
int value;
vector_pop_to(int, v, &value);   // No allocation needed
// OR
int* value = vector_pop(int, v);
vector_free_element(v, value);   // Explicit cleanup

Integer Overflow Protection

Added comprehensive overflow checking to prevent buffer overflows in memory operations:

// Before: Potential overflow
size_t size = length * element_size;  // Could overflow

// After: Safe arithmetic with validation
size_t size;
if (_safe_mul(length, element_size, &size) == -1) {
    return -1;  // Overflow detected
}

Thread Safety Issues

Fixed incorrect Windows SRWLOCK unlock behavior and enhanced cross-platform compatibility:

// Before: Always assumed exclusive lock
ReleaseSRWLockExclusive(&vec->rwlock);

// After: Proper shared/exclusive unlock handling
vector_unlock_shared(vec);   // For read operations
vector_unlock_exclusive(vec); // For write operations

Comprehensive Testing Framework

Added Unity-based testing with 15+ test cases covering:

• Memory management: Leak detection and safe cleanup patterns
• Bounds checking: Out-of-bounds access protection validation
• Thread safety: Concurrent operations with 400+ operations across 4 threads
• Edge cases: Empty vectors, NULL pointers, large allocations
• Type safety: Structs, pointers, and different data types

All security tests pass, validating the effectiveness of the implemented fixes.

Enhanced Documentation

• SECURITY_AUDIT.md: Complete vulnerability analysis and remediation details
• USAGE.md: Secure coding patterns and best practices
• Makefile: Build system with security testing targets (AddressSanitizer, ThreadSanitizer)

Compatibility and Standards

• Maintains C99 standard compliance
• Enhanced pthread compatibility across Linux/Windows/macOS
• Preserves type-agnostic design while improving safety
• No breaking changes to existing API

The library is now production-ready with comprehensive security validation and can be safely used in memory-constrained and multithreaded environments.

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