Reuse tokenizer.readBalancedBraces string/comment skipping (extracted as
skipLiteralOrComment) for both section detection and slicing, instead of a
second hand-rolled brace counter. Fixes truncation on braces inside strings
and false-positive sectioned detection from keywords inside comments/strings.
Also switch api-sections.ts errors from plain Error to LexError so parser.ts
upgrades them to ParseError with an offset.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>