Reject reversed ignore character ranges

[Widthdom]

Summary

• Reject reversed ignore character-class ranges such as [z-a] during .gitignore / .cdidxignore parsing.
• Preserve the existing malformed-rule warning path and add coverage for the specific reversed-range diagnostic.
• Add bilingual changelog fragment changelog.d/unreleased/1624.fixed.md.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests.ScanFilesDetailed_SkipsMalformedIgnoreRulesWithoutAborting"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests"
• dotnet build CodeIndex.sln
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Full dotnet test CodeIndex.sln was attempted earlier but was stopped after running for more than 17 minutes alongside other long-running test hosts; the focused scanner coverage and build passed.

Documentation / Changelog

• Added changelog.d/unreleased/1624.fixed.md.
• No README or guide update was needed because this aligns malformed ignore-rule handling with the existing warning behavior.

Fixes #1624