Clamp Markdown root-heading ranges to the actual file

[Widthdom]

Summary

CHANGELOG has 12,648 lines but its root heading is indexed with end_line:12649, exceeding the source file.

Audit IDs: R-14. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

No exact existing issue was found.

Expected behavior and PR scope

Markdown heading ranges must respect one consistent inclusive/exclusive convention and never exceed the actual last source line. Keep this to heading EOF/range calculation.

Implementation guide for Codex 5.6 sol xhigh

• Define behavior for newline-terminated versus non-terminated files and convert parser offsets once.
• Cover root and nested headings, empty/one-line files, final headings with/without body, CRLF/LF, and very large files.
• Verify symbols, outline, excerpt/definition ranges, and LSP conversion.
• Avoid special-casing CHANGELOG size/name.
• Add focused positive/negative regression tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• No heading end line exceeds total lines.
• Root/final heading bodies include exactly the intended lines.
• Line-ending variants produce equivalent logical ranges.