Add `FirecrackerGateway`, the Firecracker implementation of the shared
`Gateway` service — completing the trio (docker, macOS, firecracker) behind one
uniform contract. The gateway here is a whole microVM, so the adapter composes
`infra_vm`'s VM primitives (boot, SSH secret push, netpool links) into the ABC
surface: `connect_to_orchestrator(url, token)` parses the orchestrator guest IP
off the URL and boots the gateway VM seeding the pre-minted token; `address()`
is the gateway link's guest IP; `ca_cert_pem()` fetches over SSH (adapting the
running VM when this process didn't boot it); `provisioning_transport()` is the
SSH exec/cp transport.
Trust-model alignment with the other backends: minting moves out of
`_push_gateway_jwt` to the host composition point — `ensure_running` mints the
role-scoped `gateway` JWT and threads it through `boot_gateway(orch_ip, token)`,
so the push step only writes the token the gateway presents (#469).
`infra_vm` keeps driving the orchestrator+gateway pair as a singleton and gains
gateway-only helpers (`gateway_running`/`stop_gateway`/`gateway_guest_ip`/
`adopted_gateway_vm`); the consolidated launch reads the gateway's transport +
CA off the service.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
didericis
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