feat: map code block source ranges

[luojiyin1987]

Closes #67.

What changed

• Add source mappings for fenced and indented block-code value fields.

• Support MarkdownSourceMap.getSourceRange() with MarkdownCodeNode.

• Exclude syntax that does not belong to code.value, including:

  • opening and closing fences;
  • language, info, and meta strings;
  • stripped fenced-code indentation;
  • four-column indented-code prefixes;
  • blockquote and list container prefixes.

• Preserve LF, CR, CRLF, blank lines, trailing-line behavior, tab indentation, and per-code-unit mappings.

• Support fenced and indented code nested inside blockquotes, lists, and blockquote-list combinations.

• Map empty fenced-code values to an accurate content insertion point.

• Detect post-parse mutations of mapped code.value fields and throw SourceMapConsistencyError.

• Validate reconstructed source content against the parsed value before accepting a mapping, avoiding fabricated source ranges.

• Update the public API report, README, CHANGELOG, and package type-contract tests.

Implementation notes

Code-block mappings are reconstructed from each node’s original source position and parsed value.

The implementation removes container prefixes, fences, and parser-stripped indentation on a line-by-line basis. A mapping is accepted only when the reconstructed source content exactly matches node.value; otherwise, source mapping remains unavailable rather than returning an incorrect range.

Internal source-map segment types remain private.

Test coverage

Added coverage for:

• fenced code with LF, CR, and CRLF;
• backtick and tilde fences;
• info and meta strings;
• opening-fence indentation;
• indented code with spaces and tabs;
• blank lines and trailing line endings;
• blockquote-contained fenced and indented code;
• list-contained fenced and indented code;
• combined blockquote and list containers;
• empty fenced-code content points;
• whole-value and per-code-unit ranges;
• modified code.value consistency errors;
• AST parity between parseMd() and parseMdWithSourceMap().

Validation

• pnpm run lint
• pnpm run build
• pnpm run test
• GitHub Actions on Node.js 20, 22, and 24
• API Extractor report check
• package type-contract tests
• source-map benchmark smoke test on Node.js 22
• Source-map Core compatibility workflow against the packed candidate: passed