Harden install.sh replacement with atomic staging

[ryanfowler]

Summary

• Stage the downloaded binary on the destination filesystem, validate it, and replace the target with a single atomic rename
• Allow overriding the install directory with FETCH_INSTALL_DIR
• Add installer integration tests for replacement, cleanup, and directory/symlink edge cases
• Update CI and contributor guidance to include the install test suite

Testing

• Added integration coverage for atomic replacement and cleanup behavior in tests/install.rs
• Not run (not requested)