ci: consumer validation for actions/native-overlay fix

[KiKaraage]

Status: validation-only. Do not merge.

This PR exists solely to satisfy the consumer validation protocol for projectbluefin/actions#443, which sets native-overlay: "true" in reusable-build.yml. It carries one empty commit (GitHub rejects zero-diff branches) and changes nothing in bluefin. Once actions#443 merges and @v1 advances, this PR gets closed.

Why bluefin needs no content change for the fix

Bluefin's build workflow calls projectbluefin/actions/.github/workflows/reusable-build.yml@v1. The fix lives entirely in the actions repo; bluefin picks it up on the next Testing Images run after @v1 advances. Nothing in bluefin's Containerfile, Justfile, or workflows participates in the storage-driver decision.

What follows this PR (separate PR, not this one)

A follow-up PR against testing will, after actions#443 merges and @v1 advances:

1. Revert 010bc0f (checkpoint-rpmdb.sh, its Containerfile wiring, and its bats tests). The probe matrix shows the script is unnecessary once the builder uses kernel overlayfs: checkpoint=0 + native-overlay passes byte-identical. Ordering matters: reverting before @v1 advances would leave builds on fuse-overlayfs with no workaround.
2. Rewrite docs/skills/ci/references/failure-modes.md: retire the WAL-journal-mode theory (disproven; the no-checkpoint native leg passed with the rpmdb still in WAL mode across the stage boundary), document fuse-overlayfs 1.16 in buildah's stage commit/restore path as the root cause, and record the sha256-fingerprint probe method.

Evidence

The full bisect (probe Containerfile, sha256 fingerprints, the 2x2 driver-x-checkpoint matrix, run links) is in the actions PR: projectbluefin/actions#443.

CI

Consumer validation run: https://github.com/projectbluefin/bluefin/actions/runs/33406727723 (green; E2E smoke skips on PRs by design).

[hanthor]

This is an empty-commit consumer-validation artifact for projectbluefin/actions#443 (adds native-overlay: "true" to reusable-build.yml), per the org's documented consumer-validation protocol — explicitly marked "do not merge" and gets closed once actions#443 merges and @v1 advances. Confirmed it changes nothing in bluefin itself and that the linked consumer CI run (33406727723) is green (validate, Unit tests, Check PR base branch all pass; E2E smoke skips on PRs by design, as expected). Approving as a correctly-executed validation step, not as a substantive code change.

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