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/holdings.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"sort"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// recordDerivedHolders writes down who holds each mesh-scoped seat that nobody was ever recorded
+// as holding.
+//
+// **A seat held by derivation is a seat held by accident of being alone** (novox/hq
+// 04-ISSUES/170). ADR 0131 lets a holder on record settle a seat, and lets any other assignment
+// whose module could hold it stand beside the holder, eligible and silent. But a seat nobody
+// ever handed over has no record, so its holder is whichever assignment happened to be the sole
+// claimant — and the day a second one is assigned, both claim, both are refused, and the first
+// one's whole machine stops resolving. That is what assigning a second postgres did to the
+// control plane's own store.
+//
+// So the mesh writes the derived answer down before it acts on an assignment: for every
+// mesh-scoped seat with exactly one resolved holder and nothing on record, that holder is
+// recorded as the standing one — the same record `seat <name> --to <node>/<module>` makes by
+// hand, made from what the mesh already resolved. A seat with two derived claimants is left
+// alone: that is the ambiguity a person settles, and recording either would be guessing.
+//
+// Node-scoped seats are untouched: a record is one holder per seat, and a node-scoped seat has
+// one holder per machine (ADR 0121), so there is nothing for a record to settle there.
+func recordDerivedHolders(ctx context.Context, open *stores) ([]string, error) {
+	inv := open.inventory
+	shelf, err := inv.Catalogue(ctx)
+	if err != nil {
+		return nil, err
+	}
+	// exclude nobody: every node's claims, resolved with the holdings on record.
+	world, err := theRestOfTheMesh(ctx, inv, shelf, "")
+	if err != nil {
+		return nil, err
+	}
+	recorded, err := inv.Holdings(ctx)
+	if err != nil {
+		return nil, err
+	}
+	// A record is a row against a seat the store knows. A seat it does not — a mesh whose seats
+	// were never seeded, a seat a module declares for itself — stays held by derivation, as it
+	// always was; a missing row is not a reason an assignment fails.
+	known, err := inv.Seats(ctx)
+	if err != nil {
+		return nil, err
+	}
+	recordable := map[string]bool{}
+	for _, s := range known {
+		recordable[s.Name] = true
+	}
+	onRecord := map[string]bool{}
+	for _, h := range recorded {
+		if s, ok := catalogue.SeatNamed(h.Claim); ok {
+			onRecord[s.Name] = true
+		}
+	}
+	holders := map[string][]catalogue.Held{}
+	for _, h := range world.Held {
+		if h.Scope != catalogue.ScopeMesh {
+			continue
+		}
+		s, ok := catalogue.SeatNamed(h.Claim)
+		if !ok || onRecord[s.Name] || !recordable[s.Name] {
+			continue
+		}
+		holders[s.Name] = append(holders[s.Name], h)
+	}
+	names := make([]string, 0, len(holders))
+	for name := range holders {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	var said []string
+	for _, name := range names {
+		if len(holders[name]) != 1 {
+			continue
+		}
+		h := holders[name][0]
+		if err := inv.HoldSeat(ctx, name, catalogue.ScopeMesh, h.Node, h.Module); err != nil {
+			return said, err
+		}
+		said = append(said, fmt.Sprintf(
+			"recorded %s on %s as the standing holder of %s, which it held only by being alone",
+			h.Module, h.Node, name))
+	}
+	return said, nil
+}