fix: complete the unpaired-machine sweep + regression test

Full sweep of every machine_npub deref found one more reachable crash:
_record_rejected (tasks.py) logs machine_npub[:12], and the
assert_nostr_attribution guard now routes an unpaired machine there, so
None[:12] -> TypeError. Fall back to machine.id.

Every other deref is safe by the attribution-gate invariant: a settlement only
flows past assert_nostr_attribution (now rejecting unpaired) for a paired
machine, so the downstream distribution / parse-path / "landed" logs can't see
None; the collision-loop display already uses `(m.machine_npub or m.id)`.

Adds tests/test_unpaired_machine_guards.py: attribution rejects an unpaired
machine with the domain SettlementAttributionError (not AttributeError), and
build_state_d_tags skips it. New tests + every guard-affected suite pass.

(Two pre-existing test_pair_endpoint failures — #29 drift: fake_pair lacks
bunker_relay, and the test DB lacks super_config — are out of scope; filed
separately.)

tests/test_pair_endpoint.py

@@ -66,15 +66,7 @@ def _wire(monkeypatch, *, pair="ok"):
     async def fake_owned(machine_id, user_id):
         return _machine()
 
-    async def fake_pair(
-        machine,
-        *,
-        relays,
-        admin_client,
-        bunker_relay=None,
-        keystore_passphrase=None,
-        duration_hours=None,
-    ):
+    async def fake_pair(machine, *, relays, admin_client, duration_hours=None):
         if pair == "error":
             raise PairingError("boom")
         return _result()