feat(shaders_lab): DAG pipeline mode with node catalog

- cpp/functions/gfx/dag_types: DagStep, DagNodeDef, DagControl (header-only)
- cpp/functions/gfx/dag_catalog: 10 hardcoded nodes (4 gen, 3 op, 3 blend) ported from shader-dag-blends.jsx
- cpp/functions/gfx/dag_compile: pipeline → GLSL 330 core with fan-in via source_id
- cpp/functions/gfx/dag_uniforms: upload u_params[16] via glUniform4fv
- cpp/functions/gfx/dag_panel: ImGui pipeline editor (add/remove/reorder/controls)
- main.cpp: Code/DAG mode toggle, per-mode compile path and uniforms
- gl_loader: +glUniform4fv
- rebuild Windows .exe

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cpp/functions/gfx/gl_loader.h

@@ -36,6 +36,7 @@
     extern PFNGLUNIFORM2FPROC           fn_glUniform2f;
     extern PFNGLUNIFORM3FPROC           fn_glUniform3f;
     extern PFNGLUNIFORM4FPROC           fn_glUniform4f;
+    extern PFNGLUNIFORM4FVPROC          fn_glUniform4fv;
     extern PFNGLUSEPROGRAMPROC          fn_glUseProgram;
 
     #define glAttachShader          fn_glAttachShader
@@ -66,6 +67,7 @@
     #define glUniform2f             fn_glUniform2f
     #define glUniform3f             fn_glUniform3f
     #define glUniform4f             fn_glUniform4f
+    #define glUniform4fv            fn_glUniform4fv
     #define glUseProgram            fn_glUseProgram
 #else
     #define GL_GLEXT_PROTOTYPES