Native function registration enhancement

## Feature Request

### Description of Problem:
Currently, the `rune_attempt_register_native_functions` will set or overwrite the native function implementation.
As the initialisation is lazy, it could be misleading if a developer attempts to specify own native function implementation and the n later due to an import the stub redefines the custom installed native function to the system default one.

### Potential Solutions:
`rune_attempt_register_native_functions` to check if the function is already registered and do not override if it is.

Start by locating rune_attempt_register_native_functions and tracing how lazy initialization and imports reach it. Check how an existing native registration is represented, then verify that a custom implementation remains unchanged after a later import; add or run focused coverage for both first registration and attempted re-registration.

索引モデルが issue の本文から書いたものです。