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/delivery/preview.go

@@ -107,6 +107,8 @@ type PreviewModule struct {
 	// Paths are, of a module whose every artifact names its paths, each named path's git object id at the merge; Tree
 	// is then their NamedTree (novox/hq ADR 0499 clause 2). Absent otherwise.
 	Paths map[string]string `json:"paths,omitempty"`
+	// Packages are the mesh packages the module's lock file names at the merge (MeshPackagesOf, novox/hq ADR 0573 §2).
+	Packages []string `json:"packages,omitempty"`
 }
 
 // Preview is the value of a preview key: the repository, the head and the trunk commit it was merged onto, the merge