Add release verification tooling

[mtg0001]

Summary

• Add scripts/verify-release.sh to download release assets, verify SHA256SUMS.asc when present, and check every asset listed in SHA256SUMS.
• Add scripts/make-release-checksums.sh so maintainers can create SHA256SUMS and optionally sign it as SHA256SUMS.asc.
• Document the user and maintainer release verification flow.
• Wire the tooling through make checksums, make sign-checksums, and make verify-release.

Why

SHA256SUMS proves a download matches the checksum file, but it does not prove who wrote that checksum file. A detached signature over SHA256SUMS gives releases a clearer chain of custody: users can verify the signer first, then verify the binaries.

This is not reproducible builds yet. It is the smaller release-hardening step that makes future assets auditable before users run them.

Testing

• bash -n scripts/verify-release.sh scripts/make-release-checksums.sh
• make verify-release TAG=v1.2.13
• temporary release-asset fixture with scripts/make-release-checksums.sh followed by sha256sum -c SHA256SUMS
• scripts/verify-release.sh v1.2.13 --require-signature fails as expected because v1.2.13 does not publish SHA256SUMS.asc
• git diff --check