Fix changed-between dry-run range handling

[Widthdom]

Summary

• Added regression coverage that cdidx index --changed-between purges indexed rows for files deleted between refs, including +...md changelog fragment paths.
• Updated dry-run changed-between candidate resolution so git range/ref failures return a usage error instead of silently producing an empty candidate set.
• Stabilized retry cancellation tests after macOS/net9.0 CI exposed timing variance in the cancellation timer.
• Added bilingual changelog fragments for both issues.

Validation

• dotnet build -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Run_UpdateMode_WithChangedBetween_RemovesDeletedPath_2987|FullyQualifiedName~Run_DryRun_WithChangedBetweenInvalidRef_ReturnsUsageError_3046" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~OpenSqliteConnectionWithRetry_CancelDuringRetrySleep_ThrowsOperationCanceled|FullyQualifiedName~RegisterConnectionFunctionsWithRetry_CancelDuringRetrySleep_ThrowsOperationCanceled" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net9.0 --filter "FullyQualifiedName~OpenSqliteConnectionWithRetry_CancelDuringRetrySleep_ThrowsOperationCanceled|FullyQualifiedName~RegisterConnectionFunctionsWithRetry_CancelDuringRetrySleep_ThrowsOperationCanceled" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• changelog.d/unreleased/2987.fixed.md
• changelog.d/unreleased/3046.fixed.md
• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md edits.

Notes

• Delta indexing should purge files deleted between refs #2987 did not reproduce on the fetched latest origin/main; the PR locks the expected delete-purge behavior with regression coverage.
• No additional follow-up issues were found or created.

Fixes #2987
Fixes #3046