> 3. Values follow a strict rule: primitives pass by value, containers pass by read-only reference. This prevents accidental aliasing/mutation across scopes and keeps ownership implicit but predictable.

There are plenty of languages where functions cannot mutate their parameters or anything their parameters reference — Haskell is one example. But these languages tend to have the ability to (reasonably) efficiently make copies of most of a data structure so that you can, for example, take a list as a parameter and return that list with one element changed. These are called persistent data structures.

Are you planning to add this as a first-class feature? This might be complex to implement efficiently on top of C++’s object model — there’s usually a very specialized GC involved.