Read a rebuild of an unchanged source as no move, whatever image digest it made (hq issue 280)

An image is not byte-reproducible, so ADR 0236's 'same artifacts is no move'
never held for one: a catalogue merge that did not touch the bus rebuilt it,
and every send to the control node waited for a planned bus upgrade.

The builder now records a source fingerprint per build (module tree, context
trees, bases and toolchains by digest). A rebuild with the fingerprint of the
build it repeats is registered with that build's artifacts, handed to modules
standing on it, holds no push, demands no bus step, and a plan sends and
gates nothing for it. Identical artifacts remain a second way to be no move.

cmd/mesh-builder/main.go

@@ -292,6 +292,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 				})
 			}
 			result.Against = built.Against
+			result.SourceFingerprint = built.Source
 			for _, r := range built.Read {
 				result.Read = append(result.Read, link.ReadRepository{Repository: r.Repository, Ref: r.Ref})
 			}