Publish Spark installer r5 metadata

[vibeforge1111]

Summary

• update hosted installer scripts to spark-cli-public-installer-2026-05-15-r5
• point installer source at Spark CLI payload commit 2abbd1de2ce93d4ac86fbc16606a26c6ad2e57b4
• refresh checksums, release manifest, commands metadata, attestations docs, and public release references
• teach the release-surface check to treat the prior May 14 installer ref and hashes as stale

Verification

• node scripts/check-security-release-surface.mjs
• node scripts/check-docs-readiness.mjs
• node scripts/check-command-docs.mjs
• bash -n install.sh
• PowerShell parser check for install.ps1