Say which modules a walk left unchanged, so a merge that moved nothing is not read as stranded (hq issue 611)

A delivery plan made while another walk built a module reads it whole and names it; the walk, cut later,
finds none of the changed files in the build source that build recorded and moves nothing. Its record named
no module, so mesh-delivery read 'matched what runs' as 'never built', stopped the delivery and asked the
operator about a change already on every machine. The walk now says each such module in delivery.unchanged,
with why; its modules keep meaning what it builds.

cmd/mesh-controller/upgrades.go

@@ -692,9 +692,73 @@ func readsFile(e inventory.Entry, read []inventory.ReadRepository, p string) boo
 	if own := ownSource(read); own != nil {
 		return builder.SourceHolds(own, p)
 	}
+	return readsDirectory(e, p)
+}
+
+// readsDirectory is whether a changed file is in a module's directory, or anywhere for a module built from its
+// repository's root: the reading of a module with no recorded build source.
+func readsDirectory(e inventory.Entry, p string) bool {
 	return strings.Trim(e.Source.Path, "/") == "" || inside(p, e.Source.Path)
 }
 
+// unchangedOfMerge are the modules a merge changed files for under a reading without recorded build sources, but
+// whose recorded build source holds none of them, each with why (novox/hq issue 611): not built, because the build
+// the mesh holds is theirs (ADR 0267 §5). That reading — a delivery plan made while another walk built the module
+// (issue 606) — would have moved them; the walk says they matched what runs rather than leaving them unnamed.
+// A module built from the repository whose directory, or whole repository when built from its root, holds a
+// changed file, and a module packaging the repository's branch, are each read so. A merge whose changed files
+// were not all said moves every one of them: none is said here. Without acting.
+func unchangedOfMerge(m link.SourceMoved, entries []inventory.Entry,
+	read map[string][]inventory.ReadRepository) map[string]string {
+	if len(m.Paths) == 0 || m.PathsTruncated {
+		return nil
+	}
+	from, _, _ := mergeCandidates(m, entries, read)
+	touched, _, _ := touchedBy(from, entries, m, read)
+	out := map[string]string{}
+	for _, e := range from {
+		name := e.Manifest.Module
+		if ownSource(read[name]) == nil || slices.ContainsFunc(touched, func(t inventory.Entry) bool {
+			return t.Manifest.Module == name
+		}) {
+			continue
+		}
+		var in []string
+		for _, p := range m.Paths {
+			if readsDirectory(e, p) {
+				in = append(in, p)
+			}
+		}
+		if len(in) > 0 {
+			out[name] = fmt.Sprintf("its build source holds none of the %d changed file(s) in its directory (e.g. %s): "+
+				"the build the mesh holds is its build", len(in), in[0])
+		}
+	}
+	for _, e := range entries {
+		name := e.Manifest.Module
+		if sourceIs(e.Source, m) || !packagesSaid(read[name], m) || readsFrom(read[name], m) {
+			continue
+		}
+		out[name] = fmt.Sprintf("its build source in %s/%s holds none of the %d changed file(s) (e.g. %s): "+
+			"the build the mesh holds is its build", m.Owner, m.Repo, len(m.Paths), m.Paths[0])
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	return out
+}
+
+// packagesSaid is whether a module's newest trunk build said the build source it read in a merge's repository and
+// branch, as a context beside its own.
+func packagesSaid(read []inventory.ReadRepository, m link.SourceMoved) bool {
+	for _, r := range read {
+		if !r.Own && sameRepository(r.Repository, m) && (r.Ref == "" || r.Ref == m.Base) && len(r.Paths) > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 // staleIn is the modules whose recorded build source a plan has overtaken (novox/hq ADR 0267): a plan still
 // working that has yet to build one, or a plan made after that build which never built it — failed, stopped
 // or superseded. What such a module is built from is changing, or changed without a build to say so: a merge