fix(vm): reduce registry rootfs staging pressure

[pimlock]

Summary

Reduce peak disk usage while building registry-backed VM root filesystems, and keep the formatter error that explains failures instead of masking it with a missing fallback path. The rust-vm job now validates that the GitHub-hosted runner already provides the required ext4 tools without installing packages.

Related Issue

Closes #2423

Changes

• Remove compressed and extracted registry-layer staging data after all layers are merged and before the ext4 image is formatted.
• Preserve executed mke2fs/mkfs.ext4 failures over unavailable fallback candidates, with regression coverage for failure, missing-tool, and successful-fallback behavior.
• Validate mke2fs, mkfs.ext4, and debugfs on the rust-vm runner without installing e2fsprogs in the job.
• Deviations from the approved plan: none.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable; no files under e2e/ changed)
• mise exec -- cargo test -p openshell-driver-vm

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; no architecture or user-facing behavior changed)

[github-actions]

Label test:e2e applied for 3aea4c9. Open the existing run and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.