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/seats_declared.go

@@ -195,15 +195,8 @@ func CatalogueProblems(shelf Shelf) []string {
 				// The parser cannot do it — it also runs on the build machine, against whatever
 				// set that binary was compiled with.
 				seat, _ := SeatNamed(c.Name)
-				if c.At() != seat.Scope {
-					problems = append(problems, fmt.Sprintf(
-						"%s claims %s at scope %q, and %s is a %s seat",
-						module, c.Name, c.At(), c.Name, seat.Scope))
-				}
-				if seat.Delivers != "" && !providesAt(m, seat.Delivers, seat.Scope) {
-					problems = append(problems, fmt.Sprintf(
-						"%s claims %s, whose holder answers for %q, and %s does not provide %q at %s scope",
-						module, c.Name, seat.Delivers, module, seat.Delivers, seat.Scope))
+				if err := CanHold(m, seat); err != nil {
+					problems = append(problems, err.Error())
 				}
 				continue
 			}