feat(shaders_lab): per-node preview thumbnails + double-rclick delete node

- DagStep: preview_open flag (default false).
- dag_compile: emit `uniform int u_preview_target` and a series of
  early-return branches at the start of fragColor selection. -1 (default)
  falls through to the real Output-driven fragColor.
- dag_node_previews (new fn): per-node FBO keyed by editor_uid, lazy
  created. Renders each node with preview_open=true to its FBO by
  setting u_preview_target = step index. Texture exposed via
  dag_preview_texture(uid) for ImGui::Image.
- dag_node_editor: small toggle button "[+] preview"/"[-] preview" in
  each non-Output node; when open, ImGui::Image(96x64, V-flipped).
- dag_node_editor: double right-click on hovered node deletes it
  (Output is protected).
- main.cpp: dag_previews_render after Canvas DAG; dag_previews_destroy
  on shutdown.

Single GL program drives both the canvas and all thumbnails — moving
sliders never recompiles, only the topology change does.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/apps/shaders_lab/main.cpp

@@ -11,6 +11,7 @@
 #include "gfx/dag_panel.h"
 #include "gfx/dag_node_editor.h"
 #include "gfx/dag_palette.h"
+#include "gfx/dag_node_previews.h"
 #include "core/fps_overlay.h"
 #include "seed_shaders.h"
 
@@ -187,6 +188,11 @@ static void render() {
     }
     ImGui::End();
 
+    // Render per-node previews (only nodes with preview_open=true)
+    if (g_canvas_dag.program) {
+        fn::gfx::dag_previews_render(g_pipeline, g_canvas_dag.program);
+    }
+
     // --- Controls window (Code uniforms) ---
     if (ImGui::Begin("Controls")) {
         if (g_descs.empty()) {
@@ -232,5 +238,6 @@ int main() {
     fn::gfx::canvas_destroy(g_canvas_code);
     fn::gfx::canvas_destroy(g_canvas_dag);
     fn::gfx::dag_node_editor_destroy();
+    fn::gfx::dag_previews_destroy();
     return rc;
 }