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.)

tasks.py

@@ -235,7 +235,10 @@ async def _record_rejected(payment: Payment, machine: Machine, exc: Exception) -
         return
     logger.error(
         f"spirekeeper: rejected settlement {rejected.id} "
-        f"(machine={machine.machine_npub[:12]}..., "
+        # An unpaired machine (machine_npub None) reaches here now that
+        # assert_nostr_attribution rejects it — fall back to the id so the
+        # log line doesn't crash on None[:12].
+        f"(machine={(machine.machine_npub or machine.id)[:12]}..., "
         f"payment_hash={payment.payment_hash[:12]}...): {exc}"
     )