fix(vm): recover overlay journal before generation cleanup

[romainPellerin]

Summary

Recover the stopped VM's ext4 overlay before removing the previous generation's guest authentication files. Cleanup currently calls debugfs on an unrecovered journal; a subsequent start can fail with an unattached inode and e2fsck exit status 4.

Related Issue

No issue required: localized ordering fix in existing VM stop/start cleanup, with a regression test and no API or policy change.

Changes

• Run the existing recover_rootfs_image check before the first offline deletion.
• Preserve host authentication and generation markers when recovery fails so cleanup remains retryable.
• Add a regression test for an unrecoverable overlay and extend VM overlay coverage to three stop/start cycles with flushed file contents.
• Describe the recovery ordering in the VM driver README.

Testing

• The new unit test fails on the unchanged base and passes with this fix; all 194 VM driver library tests pass.
• VM overlay E2E passes on macOS arm64: three stop/start cycles preserve all 128 flushed files per cycle. The locally built driver used the v0.1.2 embedded runtime and v0.1.2 gateway/CLI.
• cargo clippy -p openshell-driver-vm --all-targets --locked -- -D warnings passes.
• E2E test Clippy, Rust formatting, changed-file SPDX checks and git diff --check pass.
• mise run pre-commit passes: unavailable because mise is not installed. The same limitation prevents the full mise run ci suite; focused checks are listed above.
• Unit tests added/updated
• E2E tests added/updated

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Relevant driver documentation updated

This fix retains the existing automatic-repair policy and fails closed on unrecoverable disks. It does not make VM termination a graceful filesystem shutdown; the persistence test explicitly flushes writes before stopping.

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