A holder by derivation is recorded before an assignment can unsettle it (hq 170)

`assign ace postgres` made the control plane's own store unresolvable:
postgres's manifest claims mesh-store, nobody was ever recorded as its
holder, and with two eligible assignments and nothing on record both
claimed and both were refused — novox's included. ADR 0110 says the
assignment holds, by a deliberate act; ADR 0131 gave the record its force
but left a seat nobody handed over held by whichever assignment happened
to be alone.

Before acting on an assignment the controller now writes the derived
answer down: every mesh-scoped seat the store knows, resolved to exactly
one holder with nothing on record, gets that holder recorded — the same
record `seat <name> --to <node>/<module>` makes by hand. The next
assignment able to hold the seat then stands beside the holder, eligible
and silent. A seat with two derived claimants is left for a person; a
seat on record is never rewritten; seats the store does not list stay
held by derivation as before. And the refusal, when it still happens,
names the handover that records the holder.

Verified: catalogue tests against the real catalogue; the cmd suite
against a store (the only failure, TestConvergingPreviewsThenChanges…,
fails identically on main).

internal/catalogue/holdings_test.go

@@ -158,3 +158,16 @@ func TestAStoreRowWithoutAProtocolKeepsTheCompiledOne(t *testing.T) {
 		t.Fatalf("the store's own columns were not kept: %+v", got)
 	}
 }
+
+func TestARefusalWithNothingOnRecordNamesTheHandover(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	elsewhere := []Held{{Claim: "mesh-broker", Scope: ScopeMesh, Node: "anchor", Module: "old-broker"}}
+
+	_, problems := checkClaims([]Manifest{newBroker()}, Node{Name: "laptop"}, elsewhere, nil)
+	if len(problems) != 1 {
+		t.Fatalf("two derived claimants across machines were not refused: %v", problems)
+	}
+	if !strings.Contains(problems[0], "`seat mesh-broker --to anchor/old-broker`") {
+		t.Fatalf("the refusal does not name the handover that records the holder: %s", problems[0])
+	}
+}