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).

cmd/mesh-controller/acts.go

@@ -46,6 +46,13 @@ func assign(ctx context.Context, open *stores, node, module string) (string, err
 		return "", err
 	}
 	defer release()
+	// **Before the new assignment can unsettle a seat somebody holds only by being alone**
+	// (novox/hq 04-ISSUES/170): what the mesh derived so far is written down, and then the
+	// assignment resolves against a record rather than against a coincidence.
+	settled, err := recordDerivedHolders(ctx, open)
+	if err != nil {
+		return "", err
+	}
 	fresh, err := open.inventory.Assign(ctx, node, module)
 	if err != nil {
 		return "", err
@@ -57,6 +64,9 @@ func assign(ctx context.Context, open *stores, node, module string) (string, err
 			node, module), nil
 	}
 	said := fmt.Sprintf("%s is assigned %s", node, module)
+	for _, line := range settled {
+		said += "\n  " + line
+	}
 	plan, _, err := planFor(ctx, open, node)
 	if err != nil {
 		// Kept, and still refused. Both halves are the answer, and the rest of the mesh is still