Test the newest merge's order, a second reopening's gaps and sub-second merge times (hq issues 348, 349)

The review of PR 179 found four paths no test held: which of two earlier
plans NewestMergeOf takes, a tie between them, gaps kept across a second
reopening in one keeper, and a merge time's fraction of a second.

internal/inventory/plans_test.go

@@ -73,3 +73,45 @@ func TestASupersededPlanIsNotOpen(t *testing.T) {
 		t.Fatalf("a superseded plan is not among the recent ones as superseded: %+v", recent)
 	}
 }
+
+// novox/hq issue 349 (review of the follow-up): the newest merge of a branch is the one merged last, whatever
+// order the plans were made in, in any state; a tie is broken by the plan made last; a release, another
+// branch and another repository are never it; and its time is kept to the nanosecond.
+func TestTheNewestMergeOfABranchIsTheOneMergedLast(t *testing.T) {
+	inv := ForTest(t)
+	ctx := t.Context()
+	t0 := time.Date(2026, 10, 9, 10, 0, 0, 0, time.UTC)
+	save := func(id, repo, branch string, merged, created time.Time, state string, release bool) {
+		t.Helper()
+		p := Plan{ID: id, Repository: repo, Branch: branch, Commit: id + "-commit", Merged: merged, Created: created,
+			State: state, Tiers: [][]string{}, Modules: map[string]*PlanModule{}}
+		if release {
+			p.Release = &PlanRelease{}
+		}
+		if err := inv.SavePlan(ctx, &p); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if _, found, err := inv.NewestMergeOf(ctx, "novox/mesh-catalog", "main"); err != nil || found {
+		t.Fatalf("a branch with no plan: %v %v", found, err)
+	}
+	// Made in the other order than merged: the later merge's plan made first, and done.
+	save("plan-later", "novox/mesh-catalog", "main", t0.Add(2*time.Minute+250*time.Millisecond), t0, PlanDone, false)
+	save("plan-earlier", "novox/mesh-catalog", "main", t0.Add(time.Minute), t0.Add(5*time.Minute), PlanRolling, false)
+	save("plan-other-branch", "novox/mesh-catalog", "release", t0.Add(time.Hour), t0, PlanRolling, false)
+	save("plan-other-repo", "novox/mesh-controller", "main", t0.Add(time.Hour), t0, PlanRolling, false)
+	save("release-1", "novox/mesh-catalog", "main", t0.Add(time.Hour), t0, PlanRolling, true)
+	save("plan-unknown", "novox/mesh-catalog", "main", time.Time{}, t0.Add(time.Hour), PlanRolling, false)
+	p, found, err := inv.NewestMergeOf(ctx, "Novox/Mesh-Catalog", "main")
+	if err != nil || !found || p.ID != "plan-later" {
+		t.Fatalf("the newest merge: %s %v %v", p.ID, found, err)
+	}
+	if !p.Merged.Equal(t0.Add(2*time.Minute + 250*time.Millisecond)) {
+		t.Fatalf("its merge time was not kept to the nanosecond: %s", p.Merged)
+	}
+	// The same merge time: the plan made last.
+	save("plan-again", "novox/mesh-catalog", "main", t0.Add(2*time.Minute+250*time.Millisecond), t0.Add(time.Minute), PlanBuilding, false)
+	if p, _, _ := inv.NewestMergeOf(ctx, "novox/mesh-catalog", "main"); p.ID != "plan-again" {
+		t.Fatalf("one merge time, two plans: %s", p.ID)
+	}
+}