fix: prevent sandbox virtio-fs fd exhaustion

[danny-avila]

Summary

I prevent the long-lived libkrun runner from exposing the language package tree through virtio-fs in default KVM deployments, avoiding host file-descriptor exhaustion under sustained package-heavy workloads.

• Route default KVM Compose builds through baked ext4 block-root targets while preserving automatic KVM_ENABLED=false legacy targets.
• Build the scalable worker-sandbox package tree into the read-only guest root disk and keep /host-packages disabled at the launcher boundary in KVM mode.
• Add Helm image and pvc package sources, default KVM installs to image delivery, preserve existing sandbox image overrides, and document the 0.3 migration.
• Retain the FD-aware liveness fallback and allow fdLivenessLimit: 0 to disable it as documented.
• Add deployment regression coverage for Compose target selection, Docker stage wiring, Helm rendering, custom image overrides, and invalid configurations.

Fixes #15

Change Type

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing

• Ran tests/block_root_package_delivery.sh.
• Ran git diff --check and bash -n tests/block_root_package_delivery.sh.
• Rendered all Compose variants with KVM enabled and disabled, plus the macOS override.
• Rendered and linted Helm image, PVC, local, and custom-image configurations.
• Ran Docker BuildKit --check for api/Dockerfile and docker/Dockerfile.worker-sandbox.

A full baked image build and sustained KVM soak were not run locally because the host lacks KVM and the image build compiles the complete language runtime catalog.

Test Configuration:

• Host: macOS
• Docker: OrbStack BuildKit
• Helm: v4.1.0
• Bun: 1.3.13

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective

[danny-avila]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: d32d0a7e83

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