Add FxNode render graph playground

Add persistent render graph loadouts, grouped node authoring, code-appending apply behavior, explicit playground lighting, and resilient worker startup.

Amp-Thread-ID: https://ampcode.com/threads/T-01a033d4-0e44-708e-af0e-6166283416ff
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>

server/app.go

@@ -163,11 +163,16 @@ func (app *application) savePlayground(response http.ResponseWriter, request *ht
 		http.Error(response, "method not allowed", http.StatusMethodNotAllowed)
 		return
 	}
-	request.Body = http.MaxBytesReader(response, request.Body, maximumCodeBytes+4096)
+	request.Body = http.MaxBytesReader(
+		response,
+		request.Body,
+		maximumCodeBytes+maximumLoadoutsBytes+4096,
+	)
 	var input struct {
-		ID    string `json:"id"`
-		Title string `json:"title"`
-		Code  string `json:"code"`
+		ID       string          `json:"id"`
+		Title    string          `json:"title"`
+		Code     string          `json:"code"`
+		Loadouts json.RawMessage `json:"loadouts"`
 	}
 	decoder := json.NewDecoder(request.Body)
 	decoder.DisallowUnknownFields()
@@ -175,7 +180,13 @@ func (app *application) savePlayground(response http.ResponseWriter, request *ht
 		writeJSONError(response, "The save request is not valid.", http.StatusBadRequest)
 		return
 	}
-	result, err := app.store.save(request.Context(), input.ID, strings.TrimSpace(input.Title), input.Code)
+	result, err := app.store.save(
+		request.Context(),
+		input.ID,
+		strings.TrimSpace(input.Title),
+		input.Code,
+		string(input.Loadouts),
+	)
 	if err != nil {
 		writeJSONError(response, err.Error(), http.StatusBadRequest)
 		return