refactor: record fenced code mappings during parsing

[luojiyin1987]

Summary

• record fenced code mappings from parser events
• preserve the standard fenced code AST behavior
• record empty fenced code offsets during the fence lifecycle
• preserve opening-indent semantics for closing fence offsets
• require source input for production and benchmark recording
• remove post-parse fenced code inference
• keep indented code mapping unchanged

Tests

• pnpm run lint
• pnpm run build
• pnpm run test
• node scripts/bench-source-map.mjs --smoke
• node scripts/bench-parse.mjs --smoke