refactor(orchestrator): uniform control-plane auth provisioning per trust domain

Hoist control-plane auth provisioning out of the per-backend launchers into
one shared contract, parameterized per trust domain (#476). Every blocking
finding in PR #471 was the same integration-bug class: each launcher
re-derived, by hand, how to generate the signing key, scope it to the
orchestrator, mint the gateway JWT, and keep the host key canonical.

Introduces `trust_domain.py`:

  * `TrustDomain` — one credential boundary (host-canonical key file + role
    set + env vars). `mint`/`verify` are scoped to the domain's roles, so a
    future host-controller domain (#468) uses its own key/verifier/roles
    rather than a `host` role on the control plane's frozenset (which the
    orchestrator key could then forge).
  * `ControlPlaneProvisioning` — the single seam answering the four
    invariants: host-canonical key, split key-vs-token credential, CLI token
    valid across co-running backends, and fail-closed (no open mode) for any
    co-located topology.
  * `Topology` — the backend declares what it is; the default is co-located +
    fail-closed, so a backend need not redeclare it.

The `Orchestrator` ABC gets `control_plane_key()` (fail-closed) and routes
`mint_gateway_token()` through the contract; docker/macOS/firecracker
orchestrators, the server (verify), and the host CLI client (mint cli) all go
through the domain instead of reading the host key directly. `orchestrator_auth`
gains an optional `roles=` arg (default unchanged) so a domain scopes its own
role set; `paths.host_signing_key(filename)` generalizes host_orchestrator_token.

Adds unit coverage for the domain boundary + provisioning invariants and a PRD
capturing the durable rationale. No change to the auth primitive's HMAC, the
plane split, or the server's documented open-mode fallback.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Closes #476

tests/unit/test_firecracker_orchestrator.py

@@ -44,7 +44,7 @@ class TestEnsureRunning(unittest.TestCase):
         vm = infra_vm.InfraVm(guest_ip="10.243.255.1", private_key=Path("/k"), vm=MagicMock())
         with patch.object(infra_vm, "boot_vm", return_value=vm) as boot, \
                 patch.object(infra_vm, "push_secret") as push, \
-                patch(f"{_ORCH}.host_orchestrator_token", return_value="host-key"), \
+                patch("bot_bottle.trust_domain.host_signing_key", return_value="host-key"), \
                 patch.object(orch, "is_running", return_value=False), \
                 patch.object(orch, "_ensure_registry_volume", return_value=Path("/reg")), \
                 patch.object(orch, "_wait_for_health"):