sec(install): checksum verification silently skipped when sha256sum unavailable

[cluster2600]

Summary

The installer script (install.sh, lines 178–198, 257–265) downloads a checksums file and verifies SHA-256 integrity. However:

1. If neither shasum nor sha256sum is available (lines 193–196), verification is silently skipped with only a warning
2. If the checksums file itself cannot be downloaded (lines 263–264), verification is skipped entirely

An attacker who can manipulate the download (MITM without HTTPS pinning) could serve a binary without the checksum file.

Impact

• Severity: Medium
• Users may install a tampered binary without realising verification was skipped.

Proposed Fix

1. Make checksum verification mandatory — fail if sha256sum is unavailable or the checksums file cannot be downloaded
2. Consider adding GPG/Sigstore signature verification (see also Add GitHub artifact attestations for release binaries #542 for artifact attestations)

Note: #542 covers GitHub artifact attestations which is complementary but separate from installer-side verification.