ci: report experimental amd64 failures without red status

[mikko-lab]

Summary

• keep the experimental linux/amd64 validation diagnostic and non-blocking
• report its known SIGSEGV as an Actions warning and step summary instead of a red commit check
• preserve failure diagnostics and artifact upload
• add a regression test for the workflow contract

Release gates

The seven required release checks are unchanged. The canonical reproducibility gate remains Linux/arm64.

Local verification

• CI workflow regression: 1/1 pass
• public snapshot tests: 7/7 pass
• public-release check: 436/436 allowlisted files
• git diff --check: pass