The check's composer composes a module at its build only where that build was made of the change's manifest (issue 1921, milestone 47/G)

The running indexer could not read 17 of the group's manifests, so the running node-engine composed those modules at
their trunk builds and said nothing; the head composer laid those builds over the change's other manifests and ace
failed on "nothing provides artifact-store, wanted by mesh-build-go" (group check 8a80e8e86858). The input a node
answers a check with now names, per module taken in from its build, the content digest of the manifest that build was
made of (ComposedHow.Manifests); the composer compares it with the case's manifest, and, for an input from a
node-engine before it, compares what the two manifests say beyond what a take-in resolves (the lead's ruling, §17).
Each module lifted either way is composed from the case's manifest, its build awaited, and named in NotJudged with
the comparison that lifted it; one that cannot compose without its build is held as its stub. The digest is taken
over one JSON form because a request through a build module arrives compacted.

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

internal/compose/input.go

@@ -16,6 +16,9 @@
 package compose
 
 import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -283,6 +286,28 @@ type ComposedHow struct {
 	// HeldBefore is each module the node holds composing without the change (the candidate's base), held by the read or
 	// blamed later for a refusal: the change refuses none of them (issue 774, review round 1).
 	HeldBefore map[string]string
+	// Manifests is, by module taken in from its build, the ContentDigest of the manifest that build was made of (its
+	// build record's manifest), filled in the input a node answers a check with: the check's composer composes a module
+	// at its build only where the change's manifest is that one (issue 1921). Nil from a node-engine before it.
+	Manifests map[string]string
+}
+
+// ContentDigest is the hex sha256 of a manifest's JSON in one form, its keys sorted and its spacing removed, its numbers
+// as written: the same for the bytes its repository holds and for those bytes carried inside a request, which JSON
+// encoding compacts on the way (a composition request through a build module), so the two are compared as one manifest.
+func ContentDigest(raw []byte) (string, error) {
+	dec := json.NewDecoder(bytes.NewReader(raw))
+	dec.UseNumber()
+	var v any
+	if err := dec.Decode(&v); err != nil {
+		return "", fmt.Errorf("compose: the manifest is not JSON: %w", err)
+	}
+	canonical, err := json.Marshal(v)
+	if err != nil {
+		return "", err
+	}
+	sum := sha256.Sum256(canonical)
+	return hex.EncodeToString(sum[:]), nil
 }
 
 // Place is where a node is, as its assignment says (inv.Overlay).