Add fast iteration tooling

nix run .#fast, .#test and .#webui-dev; see DEVELOPING.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/dev_test.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDevAssetsAreReloaded(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.CopyFS(dir, os.DirFS(".")); err != nil {
+		t.Fatal(err)
+	}
+	old := assets
+	assets = os.DirFS(dir)
+	defer func() { assets = old }()
+
+	sys := System{ConfigDir: filepath.Join(dir, "config"), StateDir: dir}
+	a := newApp(sys, newCollector(sys, ""), true)
+	h := a.routes()
+	if strings.Contains(do(h, "GET", "/login", nil, nil).Body.String(), "reloaded-marker") {
+		t.Fatal("marker already there")
+	}
+
+	login := filepath.Join(dir, "templates", "login.html")
+	data, _ := os.ReadFile(login)
+	i := strings.LastIndex(string(data), "{{end}}")
+	os.WriteFile(login, []byte(string(data)[:i]+"<p>reloaded-marker</p>"+string(data)[i:]), 0o600)
+	if !strings.Contains(do(h, "GET", "/login", nil, nil).Body.String(), "reloaded-marker") {
+		t.Fatal("an edited template is not picked up")
+	}
+
+	// A typo keeps the last good version.
+	os.WriteFile(login, []byte("{{define \"content\"}}{{ broken"), 0o600)
+	if !strings.Contains(do(h, "GET", "/login", nil, nil).Body.String(), "reloaded-marker") {
+		t.Fatal("a broken template replaced the working one")
+	}
+}