Harden span and dyn_array iterators

[Carson Radtke (carsonRadtke)]

Summary

• prevent callers from forging span and dyn_array iterator bounds
• support mutable-to-const iterator conversion and mixed-const operations
• make dyn_array iterators satisfy the C++20 random-access iterator requirements
• add compile-time regression coverage for iterator construction, state visibility, conversions, and concepts

Testing

• full MSVC C++20 build
• all 15 registered CTest tests pass