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/apps/shaders_lab/main.cpp

@@ -10,6 +10,7 @@
 #include "gfx/dag_uniforms.h"
 #include "gfx/dag_panel.h"
 #include "gfx/dag_node_editor.h"
+#include "gfx/dag_palette.h"
 #include "core/fps_overlay.h"
 #include "seed_shaders.h"
 
@@ -73,14 +74,32 @@ static void load_preset(const char* src) {
 }
 
 static void ensure_dag_default() {
+    // Seed with a Plasma connected to an Output node.
     if (g_pipeline.empty()) {
-        const fn::gfx::DagNodeDef* def = fn::gfx::dag_find("plasma");
-        if (def) {
-            fn::gfx::DagStep step;
-            step.id     = "n_default";
-            step.name   = def->name;
-            step.params = def->param_defaults;
-            g_pipeline.push_back(step);
+        const fn::gfx::DagNodeDef* plasma = fn::gfx::dag_find("plasma");
+        if (plasma) {
+            fn::gfx::DagStep s;
+            s.id     = "n_plasma";
+            s.name   = plasma->name;
+            s.params = plasma->param_defaults;
+            g_pipeline.push_back(s);
+        }
+    }
+    // Ensure there is always an Output node at the end.
+    bool has_output = false;
+    for (const auto& s : g_pipeline) {
+        const fn::gfx::DagNodeDef* d = fn::gfx::dag_find(s.name);
+        if (d && d->kind == fn::gfx::DagKind::Output) { has_output = true; break; }
+    }
+    if (!has_output) {
+        const fn::gfx::DagNodeDef* out = fn::gfx::dag_find("output");
+        if (out) {
+            fn::gfx::DagStep s;
+            s.id   = "n_output";
+            s.name = out->name;
+            s.editor_pos_x = 500.0f;
+            if (!g_pipeline.empty()) s.source_ids[0] = g_pipeline.front().id;
+            g_pipeline.push_back(s);
         }
     }
 }
@@ -181,6 +200,12 @@ static void render() {
     }
     ImGui::End();
 
+    // --- Functions palette (drag into DAG Pipeline) ---
+    if (ImGui::Begin("Functions")) {
+        fn::gfx::dag_palette();
+    }
+    ImGui::End();
+
     // --- Generated GLSL window (DAG compiled output, read-only) ---
     if (ImGui::Begin("Generated GLSL")) {
         if (g_dag_glsl.empty()) {