Fix C# primary constructor property extraction

[Widthdom]

Summary

• Restrict synthesized C# primary-constructor property symbols to positional record, record class, and record struct declarations.
• Bump the C# extractor contract so existing indexes discard stale false property rows during normal refresh.
• Add unit and CLI regression coverage for ordinary class/struct, nested/private and attributed multiline parameters, record variants, visibility, symbols, definition, outline, and incremental parity.
• Document the behavior and test ownership in the English/Japanese developer and testing guides, with a bilingual changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — passed with 0 warnings and 0 errors.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed.
• Full net8.0 suite — 11,821 passed, 9 skipped, 0 failed.
• Full net9.0 suite — 11,341 passed, 426 skipped, 0 failed.
• Codex review targeted net8.0 suite — 1,168 passed, 0 failed.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated 2 fragments.
• Repository root and workspace index health checks — healthy with no failed checks.
• Repository smoke test — BoundedReadStream.inner returns property count 0 from both symbols and definition; property outline retains only the five real properties.

Adversarial review

• Round 1 found one P2 documentation-sync issue for TESTING_GUIDE.md; fixed in f454534d2 together with the fragment affected-file list.
• Round 2: No blocking/actionable issues found.

Follow-up candidates

• None.

Fixes #5157