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.

internal/inventory/plans.go

@@ -220,6 +220,11 @@ type PlanDelivery struct {
 	// Withheld are, on a walk cut from a batch a stop walked merges again in, the modules it did not build: a
 	// stopped merge moves them, and built only as dependents at the branch they would deliver it (ADR 0299).
 	Withheld []string `json:"withheld,omitempty"`
+	// Unchanged are the modules a merge it answers changed files in the directory of, whose recorded build source
+	// holds none of them, each with why (novox/hq issue 611, ADR 0267 §5): not built, because the build the mesh
+	// holds is their build. A delivery plan that read them whole named them; this says they matched what runs,
+	// rather than that the walk left them unbuilt. Modules keeps meaning what the walk builds.
+	Unchanged map[string]string `json:"unchanged,omitempty"`
 	// Batch is the window of a batch; nil once it is cut into a walk.
 	Batch *PlanBatch `json:"batch,omitempty"`
 	// Alone says the walk walks one merge on its own commit, after a failed walk carried it in a later one: