Fix dom.Parse: short internal buffer error for korean text

[cjkangme]

Hi, thank you for creating and maintaining this great library.

I recently encountered a transform: short internal buffer error while parsing Korean web pages using markusmobius/go-trafilatura and go-shiori/go-readability, which depend on this repository.

While investigating the error with Claude Code, I found that it originates from the dom.Parse function, so I made a fix for it.

I'm not very familiar with Go, so I was cautious about making changes. However, after testing independently and cross-verifying with multiple AIs, I believe this fix should work correctly. I'd appreciate your review.

Problem

When using dom.Parse() on HTML documents containing large amounts of non-ASCII text (e.g., Korean), the following error occurs:

transform: short internal buffer

Root Cause

transform.Chain shares a single 4KB internal buffer between all transformers. When NFD normalization expands text significantly (common with Asian languages where a single character can decompose into multiple code points), this shared buffer overflows.

For example, Korean text like "삼성전자" expands significantly during NFD decomposition:

• Original: 4 characters
• After NFD: 11 characters - Each Korean syllable decomposes into 2-3 jamo characters ('삼' -> 'ㅅ', 'ㅏ', 'ㅁ')

Since transform.Chain stores intermediate results (after NFD, before the next transformer) in a shared buffer, this jamo decomposition can cause the intermediate data to exceed the 4KB buffer limit.

Solution

Use nested NewReader instead of transform.Chain:

// Before (shared 4KB buffer)
transformer := transform.Chain(norm.NFD, runes.Remove(softHyphenSet), norm.NFC)
return transform.NewReader(r, transformer)

// After (independent buffers per transformer)
r = transform.NewReader(r, norm.NFD)
r = transform.NewReader(r, runes.Remove(softHyphenSet))
r = transform.NewReader(r, norm.NFC)
return r

Each transformer now has its own independent buffer, enabling proper streaming processing without buffer overflow.

[cjkangme]

If you'd like me to add unit tests for this fix, please let me know and I'll be happy to write them.