feat: resident guest init (#333)

Get this thing going..







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<h2><a
href="https://app.greptile.com/api/retrigger?id=63134761"><picture><source
media="(prefers-color-scheme: dark)"
srcset="https://greptile-static-assets.s3.amazonaws.com/badges/RetriggerDark.svg?v=1"><source
media="(prefers-color-scheme: light)"
srcset="https://greptile-static-assets.s3.amazonaws.com/badges/Retrigger.svg?v=1"><img
alt="Retrigger"
src="https://greptile-static-assets.s3.amazonaws.com/badges/Retrigger.svg?v=1"
align="right"></picture></a>Confidence Score: 5/5</h2>

The PR appears safe to merge; all previous findings are resolved and the
latest readiness change introduces no established actionable regression.

<h3>Summary</h3>

- Establishes required guest filesystems, runtime directories, device
permissions, and service processes.
- Reports initialization and service deaths over the lifecycle channel.
- Supports launch, restart, and shutdown commands for a resident guest.
- Separates service and workload identities and configures per-launch
runtime environments.
- Removes the currently inactive nescope screenshot option and makes
capture-chain verification fail explicitly when compositor readback is
unavailable.
- Reworks the guest image around `nesinit` as PID 1 without a
distribution service manager.

<h3>Diagram</h3>

```mermaid
sequenceDiagram
    participant Host
    participant Init as nesinit
    participant FS as Guest filesystems
    participant Services as Service stack
    participant Workload

    Init->>Host: Ready(protocol version)
    Host->>Init: Boot(mount descriptors)
    Init->>FS: Establish and mount shares
    Init->>Services: Spawn services in order
    Services-->>Init: Required sockets ready
    Init->>Host: Initialized(service names)
    Host->>Init: Launch(id, exec, on_exit)
    Init->>Workload: Spawn with isolated UID/runtime
    Init->>Host: Started(id)
    Workload-->>Init: Exit status
    Init->>Host: WorkloadExited(id, status)
    Host->>Init: Launch / Restart / Shutdown
```

<sub>Reviews (4) · Last reviewed commit: ["fix(nesinit): readiness is a
socket
that..."](731d34df9d)</sub>

<!-- /greptile_comment -->

---------

Co-authored-by: DatCaptainHorse <DatCaptainHorse@users.noreply.github.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

build/scripts/mkimage.sh

@@ -15,14 +15,14 @@ set -euo pipefail
 
 IMAGE="${1:?usage: mkimage.sh <image-tag> <output-path> [size]}"
 OUT="${2:?usage: mkimage.sh <image-tag> <output-path> [size]}"
-SIZE="${3:-5G}"
+SIZE="${3:-3G}"
 
 if [[ "$(id -u)" -eq 0 ]]; then
     echo "mkimage.sh should run as yourself, not root/sudo — see the comment at the top of this script" >&2
     exit 1
 fi
 
-CONTAINER_RT="$(command -v docker || command -v podman || true)"
+CONTAINER_RT="$(command -v podman || command -v docker || true)"
 [[ -n "$CONTAINER_RT" ]] || { echo "Neither docker nor podman found in PATH" >&2; exit 1; }
 
 sudo -v   # cache credentials once, rather than prompting mid-pipeline