Write each module's composition per node, signed, with its digest and the move's judgement beside it (issue 1945, B1)

Every act that sent a declaration now writes compositions instead, by compare-and-set under the lease, and a
module's compositions are kept for its five newest fingerprints and whatever its want or have names (ADR 0539
clauses 1 to 5 and 14). A module the mesh provides itself, which no want can name, rides in the node's own.

internal/broker/composed.go

@@ -0,0 +1,98 @@
+package broker
+
+import (
+	"context"
+	"fmt"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"github.com/nats-io/nats.go/jetstream"
+)
+
+// The compositions' buckets (novox/hq ADR 0539 clauses 2 and 4): one per node, `composed-<node>`, holding that node's
+// compositions, which only that node's node-engine reads, since a composition can carry a sealed value (ADR 0488 §2's
+// reason); and `mesh-controller_composed`, holding each composition's digest and the judgement of its move, which the
+// assigner reads. The controller writes both, by compare-and-set while it holds the lease; each keeps five revisions
+// of a key. The names and layouts are mesh-sdk's (delivery.ComposedBucketFor, ControllerComposedBucket, ComposedKey).
+
+// ComposedValueBytes is the most a composition may hold: a file over 64 KiB travels by digest, so a module's
+// composition is far smaller; just under the server's largest message, which also carries the subject and headers.
+const ComposedValueBytes = 1<<20 - 8<<10
+
+// ComposedBucketBytes is a node's composition bucket's size: five revisions of every module's compositions of the node.
+const ComposedBucketBytes = 256 << 20
+
+// ComposedDigestsBytes is the size of the bucket of digests: a value of a few hundred bytes per node, module and
+// fingerprint kept.
+const ComposedDigestsBytes = 64 << 20
+
+// ComposedConfig is a node's composition bucket as the controller asserts it.
+func ComposedConfig(node string) (jetstream.KeyValueConfig, error) {
+	bucket, err := delivery.ComposedBucketFor(node)
+	if err != nil {
+		return jetstream.KeyValueConfig{}, err
+	}
+	return jetstream.KeyValueConfig{
+		Bucket: bucket,
+		Description: "the compositions of " + node + " (novox/hq ADR 0539): each module's resources for this node at one " +
+			"fingerprint, and the node's own, signed; written by the controller alone, read by this node's node-engine " +
+			"alone; deleted when outside the module's five newest fingerprints and named by no want",
+		History:      delivery.ComposedHistory,
+		MaxValueSize: ComposedValueBytes,
+		MaxBytes:     ComposedBucketBytes,
+		Storage:      jetstream.FileStorage,
+	}, nil
+}
+
+// ComposedDigestsConfig is the bucket of the compositions' digests as the controller asserts it.
+func ComposedDigestsConfig() jetstream.KeyValueConfig {
+	return jetstream.KeyValueConfig{
+		Bucket: delivery.ControllerComposedBucket,
+		Description: "each composition's digest and the judgement of its move from what the node runs (novox/hq ADR 0539 " +
+			"clauses 2 and 14): written by the controller alone, read by the assigner",
+		History:      delivery.ControllerComposedHistory,
+		MaxValueSize: 4 << 10,
+		MaxBytes:     ComposedDigestsBytes,
+		Storage:      jetstream.FileStorage,
+	}
+}
+
+// EnsureComposed creates the bucket of digests and each named node's composition bucket if absent and brings their
+// options to match. An update, never a delete.
+func EnsureComposed(ctx context.Context, js jetstream.JetStream, nodes ...string) error {
+	if _, err := js.CreateOrUpdateKeyValue(ctx, ComposedDigestsConfig()); err != nil {
+		return fmt.Errorf("asserting bucket %s: %w", delivery.ControllerComposedBucket, err)
+	}
+	for _, node := range nodes {
+		cfg, err := ComposedConfig(node)
+		if err != nil {
+			return err
+		}
+		if _, err := js.CreateOrUpdateKeyValue(ctx, cfg); err != nil {
+			return fmt.Errorf("asserting bucket %s: %w", cfg.Bucket, err)
+		}
+	}
+	return nil
+}
+
+// ComposedWrites is what the controller publishes to write a node's compositions: the bucket's own subjects.
+func ComposedWrites(node string) (string, error) {
+	bucket, err := delivery.ComposedBucketFor(node)
+	if err != nil {
+		return "", err
+	}
+	return "$KV." + bucket + ".>", nil
+}
+
+// ComposedReads is what a node's node-engine publishes to follow its wants and compositions (ADR 0539 clauses 1, 2 and
+// 9): every key of its own composition bucket, and its own wants (`want.<node>.<module>` and `want.<node>.node`) in the
+// assigner's bucket; nothing of another node's, nor the bucket of digests.
+func ComposedReads(node string) ([]string, error) {
+	bucket, err := delivery.ComposedBucketFor(node)
+	if err != nil {
+		return nil, err
+	}
+	if !safeSubject.MatchString(node) {
+		return nil, fmt.Errorf("%q cannot be part of a subject: a permission is a subject pattern, and this would widen it", node)
+	}
+	return append(readGrants(bucket, ""), readGrants(delivery.AssignerWantsBucket, "want."+node+".>")...), nil
+}