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_orchestrator_auth.py

@@ -82,5 +82,26 @@ class TestMintVerify(unittest.TestCase):
             mint(ROLE_GATEWAY, "")
 
 
+class TestCustomRoleSet(unittest.TestCase):
+    """The `roles=` seam a trust domain other than the control plane uses (#476):
+    mint/verify are scoped to the passed role set, not the module default."""
+
+    _ROLES = frozenset({"host"})
+
+    def test_round_trips_a_role_in_the_custom_set(self) -> None:
+        tok = mint("host", _KEY, roles=self._ROLES)
+        self.assertEqual("host", verify(tok, _KEY, roles=self._ROLES))
+
+    def test_mint_rejects_a_role_outside_the_custom_set(self) -> None:
+        with self.assertRaises(ValueError):
+            mint(ROLE_CLI, _KEY, roles=self._ROLES)
+
+    def test_verify_rejects_a_role_outside_the_verifiers_set(self) -> None:
+        # A validly signed token whose role isn't in the verifier's set fails —
+        # this is what keeps one domain's key from asserting another's role.
+        tok = mint(ROLE_CLI, _KEY)  # a control-plane `cli` token
+        self.assertIsNone(verify(tok, _KEY, roles=self._ROLES))
+
+
 if __name__ == "__main__":
     unittest.main()