A seat is handed over as one act, and the holder is on record

`seat <name> --to <node>/<module>` makes one assignment the holder of a seat in
the same write that removes the previous one. The row is new (migration 0039);
without one, the resolver derives the holder as it always did — the sole eligible
assignment, two refused — so nothing changes for a mesh that never hands a seat
over. With one, the recorded assignment holds and any other whose module could
hold the seat is eligible and silent: not refused, not holding. That is what lets
the next holder run beside the current one until the switch (hq design 26, design
28 task 5.3, ADR 0131).

Why: the controller finds its own bus through a seat, and the day that seat was
left with nobody in it — because two eligible holders could not coexist and the
old one's claim was taken away — the control plane looped for two hours while
every service stayed up. A handover that is never empty in between is the fix,
not a workaround for it.

`CanHold` is the one judgement of whether a module may hold a seat — claims it at
its scope, provides what it delivers, against the store's row — shared by
registration and the handover so they cannot drift apart. The holding belongs to
the assignment and goes when it does, so a seat never points at nothing running.

Tests: the resolver with and without a record, on the same and another machine,
under a former name; the store's row replaced not added, refused for an
unassigned target, removed with its assignment; CanHold's four answers and that
they follow the store. Full suite green against a real NATS and store.

internal/catalogue/holdings_test.go

@@ -0,0 +1,116 @@
+package catalogue
+
+import (
+	"strings"
+	"testing"
+)
+
+// **A seat's holder on record settles who holds it, and lets the next holder stand beside the
+// current one** (novox/hq ADR 0131, design 28 task 5.3). Until the record existed, two assignments
+// whose modules both claimed a seat were refused outright — which left no way to hand a seat over
+// without a moment where nobody held it, and the control plane finds its own bus through one of
+// these seats. That moment was the outage of 2026-09-27.
+
+func busSeatDelivering(t *testing.T, delivers string) {
+	t.Helper()
+	was := Seats()
+	t.Cleanup(func() { UseSeats(was) })
+	UseSeats([]Seat{{Name: "mesh-broker", Scope: ScopeMesh, Delivers: delivers, Decision: "test"}})
+}
+
+func oldBroker() Manifest {
+	return Manifest{Module: "old-broker", Provides: []Offer{{Name: "mesh-bus", Scope: ScopeMesh}},
+		Claims: []Claim{{Name: "mesh-broker", Scope: ScopeMesh}}}
+}
+
+func newBroker() Manifest {
+	return Manifest{Module: "new-broker", Provides: []Offer{{Name: "mesh-bus", Scope: ScopeMesh}},
+		Claims: []Claim{{Name: "mesh-broker", Scope: ScopeMesh}}}
+}
+
+// Nothing on record: exactly the old rule. One claimant holds; two are refused.
+func TestWithNoHolderOnRecordTheSoleClaimantHoldsAndTwoAreRefused(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	node := Node{Name: "anchor"}
+
+	held, problems := checkClaims([]Manifest{oldBroker()}, node, nil, nil)
+	if len(problems) != 0 || len(held) != 1 || held[0].Module != "old-broker" {
+		t.Fatalf("a sole claimant did not hold the seat: held=%v problems=%v", held, problems)
+	}
+	_, problems = checkClaims([]Manifest{oldBroker(), newBroker()}, node, nil, nil)
+	if len(problems) != 1 || !strings.Contains(problems[0], "both claim") {
+		t.Fatalf("two claimants with nothing on record were not refused: %v", problems)
+	}
+}
+
+// With a holder on record, the other eligible assignment is silent: not refused, and not holding.
+func TestTheHolderOnRecordHoldsAndTheOtherClaimantStandsBesideIt(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	node := Node{Name: "anchor"}
+	record := []Held{{Claim: "mesh-broker", Scope: ScopeMesh, Node: "anchor", Module: "new-broker"}}
+
+	held, problems := checkClaims([]Manifest{oldBroker(), newBroker()}, node, nil, record)
+	if len(problems) != 0 {
+		t.Fatalf("the assignment beside the holder was refused: %v", problems)
+	}
+	if len(held) != 1 || held[0].Module != "new-broker" {
+		t.Fatalf("the holder on record is not the one holding: %v", held)
+	}
+}
+
+// The record names a node too: an eligible module on another machine holds nothing, and its
+// machine's set still resolves.
+func TestAHolderOnRecordElsewhereLeavesThisMachinesClaimantSilent(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	record := []Held{{Claim: "mesh-broker", Scope: ScopeMesh, Node: "anchor", Module: "new-broker"}}
+
+	held, problems := checkClaims([]Manifest{oldBroker()}, Node{Name: "laptop"}, nil, record)
+	if len(problems) != 0 || len(held) != 0 {
+		t.Fatalf("a claimant elsewhere than the recorded holder was not simply silent: held=%v problems=%v",
+			held, problems)
+	}
+}
+
+// A record naming a seat's former name still applies to it after a rename (ADR 0122).
+func TestAHolderRecordedUnderAFormerNameStillHolds(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	wasAliases := aliases
+	t.Cleanup(func() { UseAliases(wasAliases) })
+	UseAliases(map[string]string{"the-broker": "mesh-broker"})
+	record := []Held{{Claim: "the-broker", Scope: ScopeMesh, Node: "anchor", Module: "new-broker"}}
+
+	held, problems := checkClaims([]Manifest{oldBroker(), newBroker()}, Node{Name: "anchor"}, nil, record)
+	if len(problems) != 0 || len(held) != 1 || held[0].Module != "new-broker" {
+		t.Fatalf("a record under the former name did not settle the seat: held=%v problems=%v", held, problems)
+	}
+}
+
+// CanHold is the one judgement registration and the handover share, against the store's row.
+func TestCanHoldJudgesClaimScopeAndWhatTheSeatDelivers(t *testing.T) {
+	busSeatDelivering(t, "mesh-bus")
+	seat, _ := SeatNamed("mesh-broker")
+
+	if err := CanHold(newBroker(), seat); err != nil {
+		t.Fatalf("a module that claims the seat and provides what it delivers was refused: %v", err)
+	}
+	noClaim := Manifest{Module: "quiet", Provides: []Offer{{Name: "mesh-bus", Scope: ScopeMesh}}}
+	if err := CanHold(noClaim, seat); err == nil || !strings.Contains(err.Error(), "does not claim") {
+		t.Fatalf("a module that never claimed the seat was allowed to hold it: %v", err)
+	}
+	wrongScope := newBroker()
+	wrongScope.Claims[0].Scope = ScopeNode
+	if err := CanHold(wrongScope, seat); err == nil || !strings.Contains(err.Error(), "scope") {
+		t.Fatalf("a claim at the wrong scope was allowed: %v", err)
+	}
+	cannotAnswer := Manifest{Module: "amqp-only", Provides: []Offer{{Name: "amqp", Scope: ScopeMesh}},
+		Claims: []Claim{{Name: "mesh-broker", Scope: ScopeMesh}}}
+	if err := CanHold(cannotAnswer, seat); err == nil || !strings.Contains(err.Error(), `does not provide "mesh-bus"`) {
+		t.Fatalf("a holder that cannot answer for the seat was allowed: %v", err)
+	}
+	// And the judgement follows the store's row, not a compiled copy.
+	busSeatDelivering(t, "amqp")
+	seat, _ = SeatNamed("mesh-broker")
+	if err := CanHold(cannotAnswer, seat); err != nil {
+		t.Fatalf("with the row saying amqp, an amqp provider was refused: %v", err)
+	}
+}