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/main.go

@@ -48,6 +48,9 @@ var dashHTML string
 //go:embed history.html
 var historyHTML string
 
+//go:embed trace.html
+var traceHTML string
+
 // dashTmpl — the monitoring read surface, server-rendered from dash.html (no JS,
 // no client fetch); meta-refresh keeps it live. html/template escapes the user
 // text in facts/nudges. Read-only: browses the append-only store via CoreAPI,
@@ -133,6 +136,9 @@ func main() {
 	mux.HandleFunc("/history", func(w http.ResponseWriter, r *http.Request) {
 		handleHistory(w, r, core)
 	})
+	mux.HandleFunc("/trace", func(w http.ResponseWriter, r *http.Request) {
+		handleTrace(w, r, core)
+	})
 	mux.HandleFunc("/api/revert", func(w http.ResponseWriter, r *http.Request) {
 		handleRevert(w, r, core)
 	})
@@ -372,6 +378,22 @@ input[type=text]{width:22rem}code{background:#f4f4f4;padding:.1rem .3rem}
 
 var historyTmpl = template.Must(template.New("history").Parse(historyHTML))
 
+var traceTmpl = template.Must(template.New("trace").Funcs(template.FuncMap{
+	"ago": func(t time.Time) string {
+		if t.IsZero() {
+			return "never"
+		}
+		return time.Since(t).Round(time.Second).String() + " ago"
+	},
+	"fmtTime": func(t *time.Time) string {
+		if t == nil || t.IsZero() {
+			return "—"
+		}
+		return t.Format("15:04:05")
+	},
+	"join": strings.Join,
+}).Parse(traceHTML))
+
 func handleHistory(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 	if core == nil {
 		http.Error(w, "history disabled (no -core)", http.StatusServiceUnavailable)
@@ -392,6 +414,24 @@ func handleHistory(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 	}
 }
 
+func handleTrace(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
+	if core == nil {
+		http.Error(w, "trace disabled (no -core)", http.StatusServiceUnavailable)
+		return
+	}
+	ctx := r.Context()
+	trace, err := core.TickTrace(ctx)
+	if err != nil {
+		log.Printf("trace: %v", err)
+		http.Error(w, "core read failed", http.StatusBadGateway)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	if err := traceTmpl.Execute(w, trace); err != nil {
+		log.Printf("trace render: %v", err)
+	}
+}
+
 func handleRevert(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 	if r.Method != http.MethodPost {
 		http.Error(w, "POST only", http.StatusMethodNotAllowed)