A module's directory is never shared code, held or not (hq issue 278)

The merge of mesh-catalog 7f99fb4a rebuilt 103 modules with the build agent in tier 0, and ADR
0236 recorded it as "a change to the build agent rebuilds most of the catalogue". The agent had
not changed: modules/showcase/index.ts had. showcase is the catalogue's reference module, held
by no machine, and its manifest was not in the merge, so whatTheMergeTouched read the file as
shared code and rebuilt everything built from the repository (88 came out byte-identical). The
agent stood first only because everything is built by it.

Whether a directory is a module is a fact of the repository at the merge commit, so the forge's
announcer now says it: module_dirs, the changed files' directories holding a module.json there,
with module_dirs_said. A changed file inside one is that module's business; only a file in no
such directory is shared. An announcer that does not say keeps the old rule. `plans` what-if
takes the same list as module-dirs.

And a regression for the open question: nothing depends on the build agent except by being
built by it, and built-by never widens a plan, so a change to the agent - manifest or program -
rebuilds the agent alone; what moved beside it is ordered after it.

cmd/mesh-controller/order_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"reflect"
 	"strings"
 	"testing"
@@ -243,3 +244,70 @@ func TestAModuleBuiltAtACommitStillFollowsItsBranch(t *testing.T) {
 		t.Error("a module following another branch was matched")
 	}
 }
+
+// novox/hq issue 278: whether a directory is a module is a fact of the repository at the merge commit,
+// and the announcer says it. A merge touching the code of a module the mesh does not hold — the
+// catalogue's reference module, whose manifest it left alone — read as shared and rebuilt every module
+// built from the repository (103 of them on 2026-10-06, 88 byte-identical). Said by the announcer, it
+// rebuilds nothing; a directory holding no manifest is shared as before; and an announcer that does not
+// say keeps the old rule.
+func TestAChangeInsideAModuleIsThatModulesHeldOrNot(t *testing.T) {
+	const repo = "http://forge.internal:20000/novox/mesh-catalog.git"
+	gitea := fromRepo("gitea", repo, "modules/gitea")
+	keycloak := fromRepo("keycloak", repo, "modules/keycloak")
+	known := []inventory.Entry{gitea, keycloak}
+	candidates := []inventory.Entry{gitea, keycloak}
+	merge := func(paths, modules []string, said bool) link.SourceMoved {
+		return link.SourceMoved{Owner: "novox", Repo: "mesh-catalog", Base: "main", Paths: paths,
+			ModuleDirs: modules, ModuleDirsSaid: said}
+	}
+	named := func(entries []inventory.Entry) string {
+		var names []string
+		for _, e := range entries {
+			names = append(names, e.Manifest.Module)
+		}
+		return strings.Join(names, ",")
+	}
+	showcase := []string{"modules/showcase/index.ts"}
+	for _, c := range []struct {
+		what string
+		m    link.SourceMoved
+		want string
+	}{
+		{"a module held by none, said", merge(showcase, []string{"modules/showcase"}, true), ""},
+		{"beside a held one's change", merge(append([]string{"modules/gitea/index.ts"}, showcase...),
+			[]string{"modules/gitea", "modules/showcase"}, true), "gitea"},
+		{"deeper inside it", merge([]string{"modules/showcase/daemon/index.ts"}, []string{"modules/showcase"}, true), ""},
+		{"a directory holding no manifest is still shared", merge([]string{"modules/lib/x.go"}, nil, true), "gitea,keycloak"},
+		{"one of two files in no module", merge([]string{"modules/showcase/index.ts", "modules/lib/x.go"},
+			[]string{"modules/showcase"}, true), "gitea,keycloak"},
+		{"the root is never a module directory", merge([]string{"tsconfig.json"}, []string{"", "/", "."}, true), "gitea,keycloak"},
+		{"not said: the old rule", merge(showcase, []string{"modules/showcase"}, false), "gitea,keycloak"},
+		{"an old announcer saying nothing", merge(showcase, nil, false), "gitea,keycloak"},
+		{"a manifest the merge removed, said or not", merge([]string{"modules/gone/module.json", "modules/gone/x.ts"}, nil, true), ""},
+	} {
+		if got := named(whatTheMergeTouched(candidates, known, c.m)); got != c.want {
+			t.Errorf("%s: rebuilt %q, wanted %q", c.what, got, c.want)
+		}
+	}
+}
+
+// What the announcer says reaches the controller as it is sent: the two fields, by their names on the
+// wire, and an older announcement without them reads as not said.
+func TestTheAnnouncerSaysWhichDirectoriesAreModules(t *testing.T) {
+	var m link.SourceMoved
+	if err := json.Unmarshal([]byte(`{"owner":"novox","repo":"mesh-catalog","merge_commit_sha":"abc",`+
+		`"paths":["modules/showcase/index.ts"],"module_dirs":["modules/showcase"],"module_dirs_said":true}`), &m); err != nil {
+		t.Fatal(err)
+	}
+	if !m.ModuleDirsSaid || !reflect.DeepEqual(m.ModuleDirs, []string{"modules/showcase"}) {
+		t.Fatalf("the announcer's word was lost: %+v", m)
+	}
+	var old link.SourceMoved
+	if err := json.Unmarshal([]byte(`{"owner":"novox","repo":"mesh-catalog","merge_commit_sha":"abc","paths":["x"]}`), &old); err != nil {
+		t.Fatal(err)
+	}
+	if old.ModuleDirsSaid || old.ModuleDirs != nil {
+		t.Fatalf("an older announcement says nothing about module directories: %+v", old)
+	}
+}