Merge main: the trunk renamed the seats and made them data

Both branches changed the seat set from the same starting point, so every number
collided and every `mesh-*` name existed twice. The trunk's numbers and names win:
this branch's records became 0129/0130 and its migrations 0037/0038, and the
hardcoded rename map gave way to the trunk's `seat_alias` table — a rename is a
row now (ADR 0122), not a recompile.

Three of my checks were wrong and the merge is what showed it:

A seat with an empty protocol is a marker, not an incomplete declaration. Most
node-scoped seats are markers — which module is this machine's packet filter —
and refusing one refused most of the set, the showcase module included. A
mistyped field name is already refused by the parser, so an empty protocol was
written as one deliberately.

A claim on a seat this manifest does not declare is not the parser's to judge. A
module may hold a seat another module declared; that is the whole reason ADR 0126
has callers name the seat and not its provider. Whether the seat exists is a fact
about the catalogue, so the refusal is at registration, where every declaration
is in view.

And a seat may share a name with the provision it delivers. `git`, the npm
registry and the artifact store still do, because renaming a delivering seat
cascades to every consumer requiring it, with a window where a holder stops
resolving mid-flight. The trunk deferred exactly those three on purpose.

Full suite green against a real NATS and store.

internal/link/enrolment.go

@@ -149,7 +149,7 @@ func (e Enrolment) Enrol(ctx context.Context, request EnrolRequest) (reply Enrol
 		}
 		if err := e.Inventory.RecordTunnel(ctx, node.ID, inventory.Tunnel{
 			Interface: request.Tunnel.Interface, Unit: request.Tunnel.Unit,
-			Config: request.Tunnel.Config, Port: request.Tunnel.Port,
+			Config: request.Tunnel.Config, Port: request.Tunnel.Port, MTU: request.Tunnel.MTU,
 			Address: request.Tunnel.Address, Range: request.Tunnel.Range,
 			PublicKey: request.Tunnel.PublicKey, Peers: peers,
 		}); err != nil {
@@ -234,7 +234,7 @@ func (e Enrolment) rekey(ctx context.Context, node inventory.Node, r Rekey) erro
 		peers = append(peers, inventory.TunnelPeer{PublicKey: p.PublicKey, Address: p.Address})
 	}
 	err := e.Inventory.Rekey(ctx, node.ID, r.Previous, r.OverlayKey, inventory.Tunnel{
-		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port,
+		Interface: r.Tunnel.Interface, Unit: r.Tunnel.Unit, Config: r.Tunnel.Config, Port: r.Tunnel.Port, MTU: r.Tunnel.MTU,
 		Address: r.Tunnel.Address, Range: r.Tunnel.Range, PublicKey: r.Tunnel.PublicKey, Peers: peers,
 	})
 	if err != nil {