Add arg_intent support for operator overloads

[coderabbitai]

Description

The bind_cxx_operator_overload_function in numbast/src/numbast/function.py currently accepts an arg_intent parameter but does not validate or apply it, silently ignoring user overrides.

Operator overloads have different argument intent expectations and require special handling separate from regular functions when implementing arg_intent support.

Action Items

Either:

• Implement intent planning for operator overloads by invoking compute_intent_plan and passing the resulting intent plan into the Function/ABI construction path (similar to non-operator/static paths), or
• Add an explicit guard to reject arg_intent overrides with a clear error message until support is implemented

Context

Related to PR #278: #278
Comment: #278 (comment)

cc @isVoid