mavweb: add rule trace page at /trace

New template showing the most recent tick's rule evaluation results
with color-coded table, expandable gate detail panels, nav links
to dash and history. Uses the TickTrace IPC method from #15.

cmd/mavweb/handlers_test.go

@@ -55,6 +55,10 @@ type fakeCore struct {
 	// for handleHistory tests
 	historyFacts []ipc.Fact
 	historyErr   error
+
+	// for handleTrace tests
+	tickTrace ipc.TickTrace
+	traceErr  error
 }
 
 func (f *fakeCore) EnableTool(_ context.Context, name string, cmd []string, destructive bool, scope string, _ time.Time) error {
@@ -131,6 +135,13 @@ func (f *fakeCore) RevertFact(_ context.Context, _ string) (int64, error) {
 	return f.revertNewID, nil
 }
 
+func (f *fakeCore) TickTrace(_ context.Context) (ipc.TickTrace, error) {
+	if f.traceErr != nil {
+		return ipc.TickTrace{}, f.traceErr
+	}
+	return f.tickTrace, nil
+}
+
 // --- GET ---
 
 func TestHandleTools_GET_RendersAndEscapes(t *testing.T) {
@@ -578,6 +589,88 @@ func TestHandleHistory(t *testing.T) {
 	})
 }
 
+// --- handleTrace ---
+
+func TestHandleTrace(t *testing.T) {
+	t.Parallel()
+
+	t.Run("nil core returns 503", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		handleTrace(rr, httptest.NewRequest(http.MethodGet, "/trace", nil), nil)
+		if rr.Code != http.StatusServiceUnavailable {
+			t.Errorf("status = %d, want 503", rr.Code)
+		}
+	})
+
+	t.Run("core TickTrace error returns 502", func(t *testing.T) {
+		core := &fakeCore{traceErr: ipc.ErrNoFact}
+		rr := httptest.NewRecorder()
+		handleTrace(rr, httptest.NewRequest(http.MethodGet, "/trace", nil), core)
+		if rr.Code != http.StatusBadGateway {
+			t.Errorf("status = %d, want 502", rr.Code)
+		}
+	})
+
+	t.Run("renders template with trace data", func(t *testing.T) {
+		now := time.Date(2025, 6, 1, 12, 0, 0, 0, time.UTC)
+		snooze := time.Date(2025, 6, 1, 13, 0, 0, 0, time.UTC)
+		core := &fakeCore{
+			tickTrace: ipc.TickTrace{
+				Now:    now,
+				Winner: "win-rule",
+				Rules: []ipc.RuleTrace{
+					{
+						RuleName:        "win-rule",
+						Severity:        5,
+						PredicateResult: true,
+						GateResult:      true,
+						GateDetail:      ipc.GateDetail{Presence: "present"},
+						WasSelected:     true,
+					},
+					{
+						RuleName:        "lose-rule",
+						Severity:        3,
+						PredicateResult: true,
+						GateResult:      false,
+						GateBlockedBy:   "quiet_hours",
+						GateDetail: ipc.GateDetail{
+							QuietHours: true,
+							Presence:   "away",
+							SnoozeUntil: &snooze,
+						},
+						WasSelected: false,
+						LostTo:      "win-rule",
+					},
+				},
+			},
+		}
+		rr := httptest.NewRecorder()
+		handleTrace(rr, httptest.NewRequest(http.MethodGet, "/trace", nil), core)
+		if rr.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200; body=%s", rr.Code, rr.Body.String())
+		}
+		body := rr.Body.String()
+		if !strings.Contains(body, "win-rule") {
+			t.Error("rendered output missing winning rule name")
+		}
+		if !strings.Contains(body, "lose-rule") {
+			t.Error("rendered output missing losing rule name")
+		}
+		if !strings.Contains(body, "quiet_hours") {
+			t.Error("rendered output missing gate blocked by")
+		}
+		if !strings.Contains(body, "5") {
+			t.Error("rendered output missing severity")
+		}
+		if !strings.Contains(body, "3") {
+			t.Error("rendered output missing severity for second rule")
+		}
+		if strings.Contains(body, "nothing fired") {
+			t.Error("rendered 'nothing fired' but a winner was set")
+		}
+	})
+}
+
 // --- handleRevert ---
 
 func TestHandleRevert(t *testing.T) {