Fix graceful Ctrl-C cancellation for index

[Widthdom]

Summary

• Wire index runs to a Ctrl-C/SIGINT cancellation source so the first Ctrl-C requests cooperative cancellation and the second can force-exit.
• Return E012_INTERRUPTED / exit 130 with a partial-progress message instead of stamping success readiness after interruption.
• Add regression coverage for the handler and interrupted index runner path.

Fixes #1809

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test

Documentation / Changelog

• Added changelog.d/unreleased/1809.fixed.md.

Review

• Adversarial review attempted with codex exec, but it failed due account usage limit.
• Manual adversarial pass found an optimize/readiness stamping hole and fixed it; second pass found no further blocking/actionable issues.

Follow-up

• See Indexer hangs on StructuralLineMasker.cs during per-file refresh #2303 for the separate StructuralLineMasker.cs per-file index hang observed while refreshing the local index.