Fix Elixir pipe and defimpl indexing

[Widthdom]

Summary

• Index Elixir pipe targets as call references for qualified and local pipe steps.
• Index Elixir defimpl blocks as protocol_impl symbols and emit protocol/type references.
• Migrate stale SQLite kind CHECK constraints so existing databases accept the new protocol_impl symbol/container kind.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Extract_ElixirPipeCalls_DetectsQualifiedAndUnqualifiedTargets|FullyQualifiedName~InitializeSchema_RefreshesLegacyKindCheckConstraints|FullyQualifiedName~Extract_Elixir_DetectsProtocolImplementations|FullyQualifiedName~Extract_ElixirDefimpl_CapturesProtocolAndImplementedTypes" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release --no-build -p:UseSharedCompilation=false fails in this local environment because git-backed tests inherit the user's global SSH signing key and cannot unlock /Users/widthdom/.ssh/id_ed25519_signing; observed failure mode is fatal: failed to write commit object.
• Adversarial review: No blocking/actionable issues found.

Fixes #1664
Fixes #1824