Name the _meta key the tool runner hands a bundle the id of the tool call in flight under (issue 1966 S4)

MetaCall, mesh/call: the id ApplyRequest.Call names, so the runner stamps
the caller it proved as the operation's author (ADR 0597 §7; the lead's
ruling on issue 1966 B1).

go/buscalls/buscalls.go

@@ -13,6 +13,11 @@ import (
 	"time"
 )
 
+// MetaCall is where in a `tools/call`'s `_meta` the tool runner hands a bundle the id of the tool call in flight: the id
+// ApplyRequest.Call names, so the runner stamps the caller it proved for that call as the operation's author (novox/hq
+// ADR 0597 §7). The runner writes `_meta`; a caller writes only `arguments`, so no caller can name another call.
+const MetaCall = "mesh/call"
+
 // The methods a bundle asks the tool runner, one per call.
 const (
 	ObjectsPut    = "mesh/objects.put"
@@ -188,8 +193,9 @@ type WorkQueueDeadLetters struct {
 // --- the ledger (ADR 0597)
 
 // ApplyRequest applies one operation to one instance of a ledger. Call is the id of the tool call the operation
-// serves, whose caller the tool runner proved and stamps as the author (ADR 0597 §7); empty, the operation carries the
-// owning module's account alone. An author the bundle writes into Body is never taken as the author.
+// serves, as the tool runner handed it under MetaCall, whose caller the tool runner proved and stamps as the author
+// (ADR 0597 §7); empty, the operation carries the owning module's account alone. An author the bundle writes into Body
+// is never taken as the author.
 type ApplyRequest struct {
 	Ledger    string          `json:"ledger"`
 	Instance  string          `json:"instance"`

go/packages_list_test.go

@@ -106,6 +106,10 @@ func TestMeshSDKHoldsOnlyWhatEveryModuleShares(t *testing.T) {
 		}
 	}
 
+	// 1966/AC5: the call an apply names is the id the tool runner hands under `_meta`'s mesh/call.
+	if buscalls.MetaCall != "mesh/call" {
+		t.Errorf("the tool call's id is handed under %q, not mesh/call", buscalls.MetaCall)
+	}
 	// 1966/AC5: an apply naming the tool call it serves, and its answer with the state behind, round-trip.
 	for _, c := range []struct {
 		name     string