perf(source-map): add value source offset index

[luojiyin1987]

Summary

• Add MarkdownSourceMap.getValueSourceIndex().
• Add the public MarkdownValueSourceIndex type.
• Return primitive source offsets without creating point or range objects.
• Use a cursor for forward scans.
• Use binary search for reverse and random access.
• Preserve ownership, mutation, bounds, and atomic-boundary checks.
• Document the API and update the API report.

Benchmark

The benchmark used Node 24.15.0.
It used 10,000 newline candidates and 50,000 queries per sample.
It ran 12 alternating A/B samples in one process.

Query	Median	Mean
getSourceRange(node, index, index)	12.65 ms	12.70 ms
sourceOffsetAt(index)	3.03 ms	3.03 ms

The median time decreased by 76.0%.
The source index was 4.17 times faster.
This benchmark measures boundary lookup only.
It does not claim a full lint runtime improvement.

Tests

• pnpm run build
• pnpm run lint
• pnpm test
• 23 test suites passed.
• 1,976 tests passed.

Security

No sensitive data is included.
No redaction was required.

Closes #121.