ci: adopt corral for boot gates

[hanthor]

Summary

• replace the raw QEMU/qcow2 disk gates in the PR and publish paths with corral
• add tests/corral/verify.yaml as the canonical boot-gate scenario, including pre-first-boot SSH provisioning
• capture corral screenshot evidence and document local QEMU/KubeVirt parity in docs/PIPELINE.md

Why

Corral owns the bootc disk build and provides the same boot-gate command locally and in CI. This removes duplicated Justfile/QEMU plumbing while keeping ISO verification on scripts/iso-e2e.sh.

Validation

• git diff --cached --check
• verified the canonical YAML and workflow references by inspection
• attempted go install github.com/tuna-os/corral@latest; the environment auto-selected Go 1.26.5 because the local Go 1.24.4 is older than corral's declared Go 1.26.3 requirement

Closes #578

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