Two reviewer findings addressed:
1. _kvm_accessible() now opens /dev/kvm with O_RDWR|O_CLOEXEC instead
of read-only. VM creation requires write access; a read-only
descriptor can satisfy KVM_GET_API_VERSION but fails at boot time.
2. status() now checks every hard prerequisite that require_firecracker()
checks at launch: guest kernel image, static dropbear binary, and
mke2fs. Previously a host with a configured TAP pool but missing
artifacts could pass status() and then fail during launch.
Regression coverage: TestFirecrackerKvmCheck gains test_kvm_open_rdwr_fails;
TestFirecrackerArtifactCheck covers each missing artifact; existing
TestFirecrackerStatus fixtures updated to stub the new checks.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
didericis-claude
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