Keep builds on the bus under their module, so no lookup reads the bucket whole (issue 1121, ADR 0427)

Milestone 5 moved 8,599 build records onto the bus under bare build ids, each
carrying its module's whole manifest. Every build lookup then read the whole
bucket, and a delivery plan's send did that per module per machine while it
held the plans and the machine's declaration: the send of build-agent never
finished, and every other act and the status waited behind it.

Builds now live under <module>.build.<id> (requests under req.<id>, id.<id>
names a build's module), with pointer keys newest, worked, held,
commit.<sha> and source.<reuse key> that move only to a newer build and are
read as hints, repaired when wrong. Manifests are kept once in the same
bucket under manifest.sha256-<hex>, so no grant the running bus lacks is
needed. The old keys are laid out once at start, idempotently, and left
unread. The calls a plan look makes on the silent clocks while it holds the
plans are bounded, since the vendored kv.Keys ignores its context.

internal/chain/bus.go

@@ -38,7 +38,7 @@ func OnTheBus(ctx context.Context, conn *nats.Conn, holds Holds) (*Chain, error)
 	if err != nil {
 		return nil, err
 	}
-	return New(builds, rollouts, assignments, holds), nil
+	return New(builds, rollouts, assignments, holds).WithManifests(builds), nil
 }
 
 // busBucket is a Backend on a key-value bucket. It calls no delete and no purge: the type has none to call (ADR 0408 §4).
@@ -85,7 +85,17 @@ func (b busBucket) Keys(ctx context.Context) ([]string, error) {
 }
 
 func (b busBucket) Watch(ctx context.Context) (<-chan Entry, error) {
-	w, err := b.kv.WatchAll(ctx, jetstream.IgnoreDeletes())
+	return b.WatchFiltered(ctx, ">")
+}
+
+func (b busBucket) WatchFiltered(ctx context.Context, filter string) (<-chan Entry, error) {
+	var w jetstream.KeyWatcher
+	var err error
+	if filter == ">" {
+		w, err = b.kv.WatchAll(ctx, jetstream.IgnoreDeletes())
+	} else {
+		w, err = b.kv.WatchFiltered(ctx, []string{filter}, jetstream.IgnoreDeletes())
+	}
 	if err != nil {
 		return nil, err
 	}