refactor: record indented code mappings during parsing

[luojiyin1987]

Summary

• record indented code mappings from parser events
• share code value segment recording with fenced code
• mirror the standard codeIndented AST handlers
• remove the post-parse code mapping fallback
• delete code-segments.ts
• remove the obsolete fallback microbenchmark

Tests

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