## Summary
Fixes a bug in napi_get_value_bigint_words where the function would
return the number of words copied instead of the actual word count
needed when the provided buffer is smaller than required.
## The Problem
When napi_get_value_bigint_words was called with a buffer smaller than
the actual BigInt size, it would incorrectly return the buffer size
instead of the actual word count needed. This doesn't match Node.js
behavior.
### Example
BigInt that requires 2 words: 0x123456789ABCDEF0123456789ABCDEFn
Call with buffer for only 1 word
- Before fix: word_count = 1 (buffer size)
- After fix: word_count = 2 (actual words needed)
## The Fix
Changed napi_get_value_bigint_words to always set word_count to the
actual number of words in the BigInt, regardless of buffer size.
## Test Plan
- Added test test_bigint_word_count that verifies the word count is
correctly returned
- Added test test_ref_unref_underflow for the existing
napi_reference_unref underflow protection
- Both tests pass with the fix and match Node.js behavior
🤖 Generated with [Claude Code](https://claude.ai/code)
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Co-authored-by: Claude <noreply@anthropic.com>
