Revert the revert of G's mesh-sdk merge (#191): G's sdk stands again over reader release 3

Main reverted G's half merge (#189, reverted by 8afc3eb in #191) and then took the reader release's S1-S5, which
copied G's code. G's form holds where they differ (process_bus.go, seats.go, state.go and their tests); the
release's kept names stay so main's callers keep building: SeatViewPrefix beside SeatReadPrefix, and QueueInvoke's
empty strings on not-ok (with its test TestQueueInvoke). The tree differs from G's head f30dee54 only by those and
the release's two tests (objects cap option, seat states and stream kinds).

Builds: design 47 G (the milestone's sdk part, as at f30dee54)

Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

go/stateobjects/ref_test.go

@@ -0,0 +1,43 @@
+package stateobjects
+
+import (
+	"strings"
+	"testing"
+)
+
+// ADR 0666 §16: a body is a reference when it is exactly `{"mesh-object": Ref}`; anything else is a value.
+func subABodyIsAReferenceOnlyWhenItIsExactlyOne(t *testing.T) {
+	good := Ref{Store: "views-objects", Object: "k." + strings.Repeat("a", 64), Size: 3 << 20, Digest: strings.Repeat("a", 64), Form: FormVersion}
+	got, ok := ReadRef(Wrap(good))
+	if !ok || got != good {
+		t.Fatalf("round trip: %+v, %v", got, ok)
+	}
+	d := strings.Repeat("a", 64)
+	rows := []struct{ name, body string }{
+		{"a value", `{"a":1}`},
+		{"a list", `[1]`},
+		{"not JSON", `nope`},
+		{"another field beside it", `{"mesh-object":{"store":"s","object":"o","size":1,"sha256":"` + d + `","form":1},"x":1}`},
+		{"a field the reference does not know", `{"mesh-object":{"store":"s","object":"o","size":1,"sha256":"` + d + `","form":1,"x":1}}`},
+		{"a digest that is no digest", `{"mesh-object":{"store":"s","object":"o","size":1,"sha256":"zz","form":1}}`},
+		{"another form", `{"mesh-object":{"store":"s","object":"o","size":1,"sha256":"` + d + `","form":2}}`},
+		{"no store", `{"mesh-object":{"object":"o","size":1,"sha256":"` + d + `","form":1}}`},
+	}
+	for _, r := range rows {
+		if _, ok := ReadRef([]byte(r.body)); ok {
+			t.Errorf("%s: read as a reference", r.name)
+		}
+	}
+}
+
+// TestABodyIsAReferenceOnlyWhenItIsExactlyOne runs the tests of this package that were merged to keep the module within its test budget (design 47 G §419), each as a subtest.
+func TestABodyIsAReferenceOnlyWhenItIsExactlyOne(t *testing.T) {
+	t.Run("ABodyIsAReferenceOnlyWhenItIsExactlyOne", subABodyIsAReferenceOnlyWhenItIsExactlyOne)
+	t.Run("ABodyOverTheMessageLimitIsSentAsAReference", subABodyOverTheMessageLimitIsSentAsAReference)
+	t.Run("ABundleReadsASentObjectThroughTheRuntime", subABundleReadsASentObjectThroughTheRuntime)
+	t.Run("AFoundationPartsSentStoreAgesFromItsSendLog", subAFoundationPartsSentStoreAgesFromItsSendLog)
+	t.Run("TheObjectToolsReadEverythingAndDropOnlyWhatIsNotLive", subTheObjectToolsReadEverythingAndDropOnlyWhatIsNotLive)
+	t.Run("ValuesOfAnySizeThroughTheRuntime", subValuesOfAnySizeThroughTheRuntime)
+	t.Run("AWriteLostLeavesNoObjectBehind", subAWriteLostLeavesNoObjectBehind)
+	t.Run("AValueLargerThanTheStoreTakesIsRefused", subAValueLargerThanTheStoreTakesIsRefused)
+}