Clamp Markdown ranges to physical source lines

[Widthdom]

Summary

• Clamp Markdown heading ranges to the last physical source line for LF/CRLF and terminated/non-terminated files.
• Omit empty body ranges, clamp unclosed fenced-code ranges to the same EOF contract, and bump the Markdown extractor contract so existing indexes refresh.
• Add extractor, persistence, outline/definition, LSP, and 12,648-line regression coverage.

Root cause

SourceLineSplitter intentionally preserves a trailing empty array entry for newline-terminated content, while Markdown EOF ranges used the split-array length as the physical line count.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — succeeded with 0 warnings and 0 errors for net8.0/net9.0.
• dotnet test ... -c Release --filter FullyQualifiedName~Issue4910 — 5/5 passed on both net8.0 and net9.0.
• Markdown-focused tests — 32/32 passed on net8.0; 28/28 passed with 4 existing skips on net9.0.
• Full Debug net8.0 suite — 11,041 passed, 7 unrelated transient failures, 7 skipped; all 7 failures passed when rerun in isolation.
• Full Debug net9.0 suite — 10,574 passed, 1 unrelated timing failure, 420 skipped; the failure passed when rerun in isolation.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -p:UseSharedCompilation=false -- check
• Real CHANGELOG.md outline smoke test — total_lines: 12648, root end_line: 12648, root body_end_line: 12648.
• Codex adversarial review — first-round unclosed-fence finding fixed; second round completed with no findings.

Documentation and changelog

• Updated the English and Japanese Markdown range contracts in DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4910.fixed.md.

Follow-up candidates

None.

Fixes #4910