SDK-9: mesh-sdk takes no new seat-specific name (ADR 0613 §5, design 26 *Contracts modules*)

Every seat constant of mesh-sdk is mesh-contracts' generated name; the four whole seats' (BrokerSeat, CheckerSeat,
asks.Seat, buscompose.VaultSeat) are marked deprecated for their importers. The package walk of mesh-sdk's own check
refuses a string literal naming a defined seat or its subjects, and an exported name given to one, beyond
testdata/seat-names.txt, which lists what stands before the move (module names that equal a seat's among them) and
only shrinks. No new test function.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

go/buscompose/users.go

@@ -1,5 +1,7 @@
 package buscompose
 
+import contractseats "git.novox.be/novox/mesh-contracts/go/seats"
+
 // Declared is one module on one node, as composing its authority needs it.
 type Declared struct {
 	Module   string
@@ -102,4 +104,6 @@ type ProcessPrincipal struct {
 }
 
 // VaultSeat is the vault's seat: its holder alone asks the desks and the issuing providers in its own name.
-const VaultSeat = "mesh-vault"
+//
+// Deprecated: a whole seat's name is mesh-contracts' (seats.MeshVault); kept until its importers take it from there.
+const VaultSeat = contractseats.MeshVault