YAML fixes

[stephenberry]

YAML Quoted String Line Folding & Block Scalar Sibling Key Fixes

Quoted String Line Folding

Implements proper YAML line folding for double-quoted and single-quoted strings per the YAML spec:

• Single newline between content lines is folded into a space
• Blank lines (double newlines) are preserved as literal newlines
• Trailing whitespace before a line break is trimmed
• Leading whitespace on continuation lines is trimmed
• Backslash continuation (double-quoted only): \ at end of line joins the next line with no space

The previous SWAR-optimized parsing in both parse_double_quoted_string and parse_single_quoted_string has been replaced with byte-by-byte processing that correctly handles embedded newlines, escape sequences, and line folding. The SWAR approach couldn't handle the multi-character newline/folding logic that the YAML spec requires within quoted strings.

Block Scalar Sibling Keys

Adds tests verifying that block scalars (|, >) followed by sibling keys at the same indentation level are parsed correctly (e.g., k1: |\n a\nk2: c).

Tests

Added comprehensive test coverage including:

• Simple and complex line folding for both quote styles
• Blank line preservation
• Backslash line continuation (double-quoted)
• Trailing whitespace trimming
• StackOverflow-style multiline examples with mixed features
• Block scalar followed by sibling keys in mappings and sequences