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/CMakeLists.txt

@@ -13,6 +13,7 @@ add_imgui_app(shaders_lab
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_panel.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_node_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_palette.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/dag_node_previews.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/fps_overlay.cpp
 )
 target_include_directories(shaders_lab PRIVATE