Fix Rust conditional derives and unsafe indexing

[Widthdom]

Summary

• Index Rust multiline cfg_attr(..., derive(...)) trait references with correct source positions.
• Index Rust unsafe { ... } blocks as scoped containers, including expression-position blocks.
• Emit Rust mutable reference target types for &mut Type while avoiding mutable-borrow expression false positives.

Changelog

• changelog.d/unreleased/2052.fixed.md
• changelog.d/unreleased/2053.fixed.md

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "RustMultilineCfgAttrDeriveAttributes|RustMutableReferenceTypes|RustMutableDynAndImplReferences|RustMutableBorrowExpression"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "Rust_DetectsUnsafeBlockContainer|Rust_DetectsUnsafeBlockInExpression"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RustMutableReferenceTypes|FullyQualifiedName~RustMutableDynAndImpl|FullyQualifiedName~RustMultilineCfgAttr|FullyQualifiedName~UnsafeBlock"
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted earlier, showed no failures while running, but was stopped after a long silent run before a final test summary.

Fixes #2052
Fixes #2053