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/request.go

@@ -2,9 +2,12 @@ package buildstep
 
 import (
 	"fmt"
+	"maps"
 	"path"
 	"regexp"
 	"strings"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
 )
 
 var (
@@ -27,6 +30,10 @@ type FetchRequest struct {
 	Base       string   `json:"base,omitempty"`
 	Dir        string   `json:"dir,omitempty"`
 	Paths      []string `json:"paths,omitempty"`
+	// Packages are the version of each mesh package the build or the check builds against (delivery.StepAsk.Packages,
+	// novox/hq ADR 0573 §2, §4): the build module writes each into the workspace's go.mod and go.sum, whatever version
+	// the repository names, checksum-verified, and names them in its log.
+	Packages delivery.PackageVersions `json:"packages,omitempty"`
 }
 
 // FetchRequestProperties are the JSON schema properties of FetchRequest's dir and paths, for a fetch tool's Input.
@@ -35,6 +42,8 @@ func FetchRequestProperties() map[string]any {
 		"dir": map[string]any{"type": "string", "description": "the module's directory in the repository (\".\" for its root), with paths"},
 		"paths": map[string]any{"type": "array", "items": map[string]any{"type": "string"},
 			"description": "the paths the module names, relative to dir, from the indexer's kept inputs: the build sees only these, the manifest and its lock files"},
+		"packages": map[string]any{"type": "object", "additionalProperties": map[string]any{"type": "string"},
+			"description": "optional: the version (a full commit) of each mesh package to build against, by its module path, whatever version go.mod names (novox/hq ADR 0573 §2); the builder and the checker fill it"},
 	}
 }
 
@@ -49,6 +58,12 @@ func (r FetchRequest) Args() map[string]any {
 	if len(r.Paths) > 0 {
 		args["paths"] = append([]string(nil), r.Paths...)
 	}
+	if len(r.Packages) > 0 {
+		args["packages"] = map[string]any{}
+		for p, v := range r.Packages {
+			args["packages"].(map[string]any)[p] = v
+		}
+	}
 	return args
 }
 
@@ -78,9 +93,42 @@ func ParseFetchRequest(args map[string]any) (FetchRequest, error) {
 	default:
 		return FetchRequest{}, fmt.Errorf("paths is a list of strings")
 	}
+	packages, err := PackagesArg(args["packages"])
+	if err != nil {
+		return FetchRequest{}, err
+	}
+	r.Packages = packages
 	return r, r.Check()
 }
 
+// PackagesArg reads a tool call's packages argument, an object of strings (as JSON gives it) or the map an ask gives;
+// nil for none.
+func PackagesArg(v any) (delivery.PackageVersions, error) {
+	out := delivery.PackageVersions{}
+	switch v := v.(type) {
+	case nil:
+		return nil, nil
+	case delivery.PackageVersions:
+		maps.Copy(out, v)
+	case map[string]string:
+		maps.Copy(out, v)
+	case map[string]any:
+		for p, version := range v {
+			s, ok := version.(string)
+			if !ok {
+				return nil, fmt.Errorf("packages is an object of versions (strings), by package")
+			}
+			out[p] = s
+		}
+	default:
+		return nil, fmt.Errorf("packages is an object of versions (strings), by package")
+	}
+	if len(out) == 0 {
+		return nil, nil
+	}
+	return out, out.Check()
+}
+
 // Check is what ParseFetchRequest refuses.
 func (r FetchRequest) Check() error {
 	if !requestRepositoryOK.MatchString(r.Repository) || strings.Contains(r.Repository, "..") {