Denotability for storing / not storing references

# Search terms
ownership, variance

#410 would benefit from knowledge regarding ownership (ie, if something keeps references to something)

```py
def foo() -> Unowned[list[str]]:
return [""] # no error
def bar(b: Owns[list[list[str]]]) -> Unowned[list[str]]:
return b[0] # error: this value is owned
result = []
b.append[result] # note: ownership taken here
return result # error: is value is owned

a: list[object] = foo() # no error
```

things will be stored by default?

Begin with issue #410 and the ownership and variance examples in this issue. Determine and specify whether references are stored by default, including the expected outcomes for foo, bar, and the list assignment. Done means the ownership behavior is clearly defined and its examples are covered by appropriate tests.

Written by the indexing model from the issue text.