Hydra OS builds start from the pinned amd64 systemd stage3 and pinned Gentoo
repository in `sources.json`. Run the builder as the normal user with Python 3,
Podman and GnuPG available; temporary tool environments are sufficient. It
refuses host root and work/cache paths outside the primary project checkout.

```
scripts/hydra-build fetch
scripts/hydra-build prepare
scripts/hydra-build packages --pretend
scripts/hydra-build packages
scripts/hydra-build rootfs
scripts/hydra-build iso
scripts/hydra-build status
```

`--work PATH`, `--cache PATH`, `--podman PATH` and `--gpg PATH` override defaults.
Linked worktrees use the primary checkout's `build/` and `cache/`. The build
lock prevents simultaneous writes in the same work directory. Interrupted
package installation resumes through Portage; successful steps write
`build/manifest.json`, signature evidence and the installed package/version/USE
inventory in `build/artifacts/`. Downloaded inputs are reverified on every
prepare; missing or changed files cannot be trusted from a checkpoint alone.

The dedicated rootless Podman store is `build/podman`, separate from default
Podman and Distrobox storage. The build container mounts inputs and repository
read-only and retains its writable layer across failures. Host networking makes
the existing local VPN/proxy available. `SYS_ADMIN` operates within the rootless
user namespace; package sandbox features incompatible with it are disabled.
No host disks or `/dev/kvm` are passed into the builder. Hydra never prunes
containers/images or changes host services.

Official Gentoo binpkgs require signatures. USE mismatches and selective testing
keywords can cause source builds; concurrency is four compiler processes and
one emerge job. Keep licenses and Portage's `/var/db/pkg`, world and repository
for installed-system updates. Rolling binhost contents can change; resulting
versions/USE are recorded without claiming bit-identical output. The pinned
service-keyring hash may need deliberate review when Gentoo rotates keys.

`rootfs` exports the dedicated builder into a private intermediate and assembles
an isolated sanitized payload, generic live initramfs and squashfs. `iso` checks
those artifacts and emits the UEFI `HYDRA_OS` ISO, SHA256 and build manifest in
`build/artifacts/`. The working builder is retained for package retries. The
installed Gentoo repository uses the standard rsync backend with signed
metamanifest verification; the builder continues to use its pinned snapshot.
QEMU and remote-viewer retain local patches and are excluded from upstream
binary packages on installed updates. Disposable boot/install validation is
required before treating an emitted ISO as release ready.

The x86_64 ISO disables GRUB's optional HFS+/APM hybrid. Otherwise that second
filesystem duplicates the `HYDRA_OS` label and can make dracut mount the wrong
live medium after raw USB writing. ISO inspection includes the system area and
rejects HFS+/APM. Test both optical and USB attachment with `dev/iso-smoke.py
--live-only --live-media cdrom` and `--live-only --live-media usb` (plus the normal
ISO and OVMF arguments); optical boot alone is insufficient.