Key expanded source by version

## Problem

Currently the expanded source is global shared state that is re-populated each time solc is run. Forge specifically documents that a source may be compiled multiple times across different solc versions. Currently, multiple runs would overwrite each other.

## Proposed solution

Expanded sources should be keyed by version so no expansions are lost

Start by locating the global expanded-source state in the Rust macro expansion pipeline and trace how it is repopulated when Forge invokes solc. Verify the behavior across repeated compilations with different solc versions; done means each version retains its own expanded sources without overwriting another version.

Written by the indexing model from the issue text.