Carry mesh package versions in build asks and fingerprints

ADR 0573 §2 has every module build against the latest published version of
each mesh package it uses, chosen by the planner and written by the build,
never pinned by hand (issue 1585, S4). The ask, the fetch request and the
merge preview now carry the packages and their versions, and the new-form
fingerprint takes each version, so a package's trunk merge changes every
dependent's fingerprint. A module using no package keeps its fingerprint.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/buildstep/askrun.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"maps"
 	"path"
 	"slices"
 	"sort"
@@ -42,14 +43,30 @@ type AskSteps struct {
 // RunAsk runs ask's job with steps and answers a delivery.BuildAnswer or a delivery.CheckAnswer.
 func RunAsk(ctx context.Context, steps AskSteps, ask delivery.StepAsk) (any, error) {
 	if ask.Build != nil {
-		return steps.build(ctx, *ask.Build)
+		job := *ask.Build
+		job.Fetch = withPackages(job.Fetch, ask.Packages)
+		return steps.build(ctx, job)
 	}
 	if ask.Check != nil {
-		return steps.check(ctx, *ask.Check)
+		job := *ask.Check
+		job.Fetch = withPackages(job.Fetch, ask.Packages)
+		return steps.check(ctx, job)
 	}
 	return nil, Refuse("the ask %s holds no job", ask.ID)
 }
 
+// withPackages is the fetch's arguments with the ask's package versions (novox/hq ADR 0573 §2), which the build module
+// writes into the workspace it fetches; a copy, so the ask is left as it was. An ask with none, or a copy alone that
+// fetches nothing, is left as it is.
+func withPackages(fetch map[string]any, packages delivery.PackageVersions) map[string]any {
+	if len(packages) == 0 || fetch == nil {
+		return fetch
+	}
+	out := maps.Clone(fetch)
+	out["packages"] = maps.Clone(packages)
+	return out
+}
+
 func raw(v any) (json.RawMessage, error) {
 	if r, ok := v.(json.RawMessage); ok {
 		return r, nil