Reduce source-backed serialization overhead

[aehlke]

Follow-up optimizations for the source-backed serializer merged in #397.

This change:

• appends contiguous byte buffers directly instead of materializing suffix collections;
• handles partially filled pooled buffers without slice allocation;
• returns immediately when a source-backed document has no dirty roots, avoiding output-settings copies and patch traversal.

The behavior and fallback policy are unchanged.

Verification:

• complete SwiftSoup suite: 662 passed, 0 failed