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.

internal/builder/builder_test.go

@@ -36,6 +36,8 @@ type recorded struct {
 	// carried timestamps would still produce one digest — which is a test that passes for a
 	// reason that has nothing to do with what it claims.
 	stamped time.Time
+	// tree is what git names as the tree of the module's directory; empty answers as for a commit.
+	tree string
 }
 
 func (r *recorded) run(_ context.Context, dir, name string, args ...string) (string, error) {
@@ -77,6 +79,8 @@ func (r *recorded) run(_ context.Context, dir, name string, args ...string) (str
 			}
 		}
 		return "", nil
+	case name == "git" && len(args) > 1 && args[0] == "rev-parse" && strings.HasPrefix(args[1], "HEAD:") && r.tree != "":
+		return r.tree + "\n", nil
 	case name == "git" && len(args) > 0 && args[0] == "rev-parse":
 		return "c0ffeec0ffeec0ffeec0ffeec0ffeec0ffeec0ff\n", nil
 	}