feat(shaders_lab): Output node + Functions palette with drag-drop

- DagKind::Output (new enum): terminal sink; compiler wires fragColor to its source_ids[0]
- dag_catalog: "output" node (1 input, red)
- dag_compile: skips Output in node_<i> emission; final fragColor resolves from Output's connection
- dag_node_editor: no more Add button; drops "DAG_NODE_TYPE" payloads at mouse canvas position; Output cannot be deleted; Output has no output pin
- dag_palette (new fn): Functions window with grouped, draggable node cards
- main.cpp: "Functions" window added; ensure_dag_default seeds plasma + connected Output

cpp/functions/gfx/dag_catalog.cpp

@@ -225,6 +225,21 @@ static const std::vector<DagNodeDef>& build_catalog() {
             v.push_back(std::move(n));
         }
 
+        // ── Output (sink — drives fragColor) ─────────────────────────
+        {
+            DagNodeDef n;
+            n.name   = "output";
+            n.label  = "Output";
+            n.desc   = "canvas DAG output";
+            n.kind   = DagKind::Output;
+            n.num_inputs = 1;
+            n.param_names    = {"", "", "", ""};
+            n.param_defaults = {0.0f, 0.0f, 0.0f, 0.0f};
+            n.controls = {};
+            n.body_glsl = [](int) -> std::string { return ""; };
+            v.push_back(std::move(n));
+        }
+
         return v;
     }();
     return catalog;