A follower joining a watched state hears its current values, and a templates archive is fetched once

A read could start a state's watch before its follower joined, and a change landing in that gap woke nothing (review
of ADR 0580 §2): a joining follower now runs over the copy's current values before any later change. The watched
reader keeps each archive once per URL, and both caches are bounded.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/compose/frombus/frombustest/frombustest_test.go

@@ -157,6 +157,16 @@ func TestTheProvisionedMeshHandsAPlanWithItsProvision(t *testing.T) {
 		byKey[frombus.PlacementRead+" watched"] != 1 {
 		t.Errorf("the watched reader read %v, want a build by key each time, a manifest once, the placement watched once", byKey)
 	}
+	fetched := 0
+	keep := fromruntime.KeptFetch(func(context.Context, string) ([]byte, error) { fetched++; return []byte("tar"), nil })
+	for range 2 {
+		if body, err := keep(context.Background(), "http://store/restic/files/blobs/sha256:a"); err != nil || string(body) != "tar" {
+			t.Fatalf("the kept fetch answered %q, %v", body, err)
+		}
+	}
+	if fetched != 1 {
+		t.Errorf("a templates archive was fetched %d times, want once", fetched)
+	}
 }
 
 func planned[P any](plans map[string]P, module string) bool { _, ok := plans[module]; return ok }