Implements a Rust napi-rs module that parses YAML-like frontmatter,
markdown sections, and roadmap structures from .gsd/ files. Provides
parseFrontmatter, extractSection, extractAllSections, batchParseGsdFiles,
and parseRoadmapFile functions exposed via @gsd/native. The JS parsers
in files.ts fall back transparently when the native module is unavailable.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
