Adds migration prompts

.github/copilot-instructions.md

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+# GitHub Copilot Repository Instructions: C++23 Modernization
+
+## Context
+- Language: C++
+- Target standard: C++23
+- Goal: modernize legacy C++11/14/17 code safely and incrementally
+
+## Primary Rule
+Preserve existing behavior unless functional changes are explicitly requested.
+
+## Balanced Modernization Policy
+- Prefer modern C++23 constructs when they clearly improve safety, readability, or maintainability.
+- Keep refactors focused and reviewable; avoid broad rewrites unless requested.
+- Prefer standard library solutions over custom or legacy patterns.
+- Respect toolchain limitations, ABI-sensitive interfaces, and repository-local style.
+
+## Preferred Modernizations
+- Use `auto` where the type is obvious from the initializer; avoid it when it hides important type information.
+- Prefer structured bindings over `std::tie` or manual pair/tuple access when clearer.
+- Replace raw owning pointers and manual `delete` with RAII, usually `std::unique_ptr`.
+- Use `std::make_unique` and `std::make_shared`; prefer `std::shared_ptr` only for real shared ownership.
+- Use `std::string_view` for read-only string parameters only when lifetime safety is clear.
+- Prefer `std::span` for non-owning contiguous ranges.
+- Use `std::mdspan` only when multidimensional access is central and the change improves clarity or safety.
+- Prefer `.contains()` over `find(...) != end()` where available and clearer.
+- Prefer standard algorithms and ranges over manual loops when readability improves.
+- Use `[[nodiscard]]` where ignoring the result is likely a bug.
+- Prefer early returns over deeply nested control flow.
+- Use `std::optional` monadic operations (`and_then`, `transform`, `or_else`) when they make the code simpler.
+- Prefer `constexpr`, `if constexpr`, concepts, and `requires` over legacy `std::enable_if` / SFINAE where practical.
+- Add `noexcept`, `override`, `final`, `= default`, and `= delete` where correct and useful.
+- Prefer `enum class` over unscoped enums where practical.
+- Prefer `std::print` / `std::println` for console output when supported by the project toolchain.
+- Prefer typed `std::chrono` durations and `std::jthread` where appropriate.
+
+## Avoid
+- Do not modernize mechanically if readability becomes worse.
+- Do not introduce unsupported C++23 features.
+- Do not introduce dangling references, dangling `std::string_view`, or unclear ownership.
+- Do not silently change ABI, object lifetime, exception behavior, or performance characteristics.
+- Do not rewrite unrelated code.
+
+## Expected Copilot Behavior
+When making migration suggestions:
+- produce small, focused updates
+- explain non-trivial changes briefly
+- call out risky refactors explicitly
+- note compatibility or toolchain caveats
+- leave optional follow-up improvements separate
+
+## Preferred Output
+Provide:
+- updated code
+- short rationale
+- compatibility/toolchain caveats
+- optional next modernization steps