Use modern language features

Before I open the refactor PR, make it easy to review the modernizations: show diffs that replace pointer arithmetic with standard algorithms, highlight where ownership becomes unique_ptr instead of raw new/delete, and annotate any places where using concepts, spans, or coroutines could simplify code but were left out for compatibility reasons. Flag changes that could alter performance or ABI.

I rewrote the tight processing loop using ranges and std::views to make it clearer, and Mary owns performance testing. The unit tests pass but our profiler shows a 10% regression in that path. I can’t tell if this is allocator behavior, iterator abstraction cost, or missed move semantics. What’s the most efficient way to diagnose whether to revert to the old loop or tune the modern code for equivalent performance?

We repeatedly convert older code to newer language features and then find subtle performance regressions, ABI incompatibilities, or harder-to-debug call stacks. The pattern is doing transformations without a checklist for risk. Which practices should we adopt—tooling, benchmarks, and review principles—so we get the readability and safety benefits of modern features without sacrificing performance or stability?