review: hold the hosts region through composition, and say so in plan

A declaration-level test composes the shipped networking module with a
resolver and asserts /etc/hosts arrives as mesh-wireguard.fact-node-names
with into: block and region-only content, that the resolver's restart-on
still names it, and that a resource's at passes through untouched — a
composition step dropping into would otherwise go unnoticed. plan --show
marks files written into, so a region is not read as the whole file.
The rollout order is spelled out: every node's host, the controller's
own included, must be block-aware before this controller ships (hq 128).

cmd/mesh-controller/plan_test.go

@@ -35,3 +35,17 @@ func TestListensLinesAreEmptyForAModuleWithNothingToListenOn(t *testing.T) {
 		t.Errorf("a module with no listens should print nothing, got %v", got)
 	}
 }
+
+// `plan --show` says when a file is written into rather than over (novox/hq issue 128), or the
+// mesh's region of a hosts file reads as the whole file.
+func TestAFileWrittenIntoIsShownAsSuch(t *testing.T) {
+	region := shownAs(map[string]any{
+		"id": "mesh-wireguard.fact-node-names", "path": "/etc/hosts", "into": "block"})
+	if region != "mesh-wireguard.fact-node-names  /etc/hosts (written into, block)" {
+		t.Errorf("the region is shown as %q", region)
+	}
+	whole := shownAs(map[string]any{"id": "dnsmasq.fact-node-zones", "path": "/etc/mesh-resolver/nodes.conf"})
+	if strings.Contains(whole, "written into") {
+		t.Errorf("a whole file is shown as written into: %q", whole)
+	}
+}