Summary

• clarify Steam behavior wording: macOS usually does not show install, Linux may allow install via Steam Play/Proton
• add a Linux-specific option for reusing existing Steam/Proton installs
• document environment-variable examples for running without copying assets
• align installation guide prerequisite wording with the updated behavior
• add dev diary entry for this documentation session

Why

Issue #132 reported that Linux users can install through Steam/Proton, so the previous docs statement was too strict and potentially misleading.

Validation

• reviewed updated docs for consistency between game-files and installation guides
• kept scope documentation-only

Fixes #132