Fix critical buffer overflow vulnerability in fromBigInt function

[shadowmystical3-ai]

• Fixes CVE security issue: Buffer overflow in bigint-buffer native module
• Problem: The stack allocation check only validated word count (word_width_bytes) but the actual memset/malloc used byte_width + overflow_len, causing overflow when large numbers were converted with non-64-bit alignment
• Solution: Changed the stack-fit check to validate the actual allocation size (buffer_size = byte_width + overflow_len) before deciding between stack and heap
• Impact: Prevents potential crash/exploit attacks when processing large BigInt values with non-aligned word widths
• Testing: All existing tests pass with compiled native module