Say each build's source fingerprint in builds, the one tool that reads what the builder recorded (issue 1204)

The builder records a build's source fingerprint, but no tool read it back, so design 47 R3a could not
reproduce a recorded one (ADR 0437 clause 5).

cmd/mesh-controller/build_test.go

@@ -1,9 +1,12 @@
 package main
 
 import (
+	"bytes"
+	"strings"
 	"testing"
 	"time"
 
+	"github.com/novox/mesh-controller/internal/inventory"
 	"github.com/novox/mesh-controller/internal/link"
 )
 
@@ -32,3 +35,34 @@ func TestABuildIDSaysWhenItWasAsked(t *testing.T) {
 		}
 	}
 }
+
+// novox/hq issue 1204: builds says the source fingerprint the builder recorded, the one tool that reads it.
+func TestBuildsSaysTheSourceFingerprint(t *testing.T) {
+	at := time.Date(2026, 10, 19, 10, 40, 0, 0, time.UTC)
+	built := inventory.Build{ID: "build-1", Module: "mailu", Commit: "b4527d9091e972f28cb365623d9280045e7741a3", On: "ace",
+		Repository: "novox/mesh-catalog", At: at, ReuseKey: "r1:abc",
+		SourceFingerprint: "src1:0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"}
+
+	// 1204/AC1
+	var out bytes.Buffer
+	printBuild(&out, built)
+	lines := strings.Split(strings.TrimRight(out.String(), "\n"), "\n")
+	if last := lines[len(lines)-1]; last != "    source     "+built.SourceFingerprint {
+		t.Errorf("last line %q; want the source line after the reuse key:\n%s", last, out.String())
+	}
+	if !strings.Contains(lines[len(lines)-2], "reuse key  r1:abc") {
+		t.Errorf("the source line does not follow the reuse key:\n%s", out.String())
+	}
+
+	// 1204/AC2
+	without := built
+	without.SourceFingerprint = ""
+	var plain bytes.Buffer
+	printBuild(&plain, without)
+	if strings.Contains(plain.String(), "source") {
+		t.Errorf("a build with no fingerprint says a source line:\n%s", plain.String())
+	}
+	if want := strings.Join(lines[:len(lines)-1], "\n") + "\n"; plain.String() != want {
+		t.Errorf("the other lines changed:\n%s\nwant\n%s", plain.String(), want)
+	}
+}