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.cpp

@@ -30,6 +30,7 @@ PFNGLUNIFORM1IPROC           fn_glUniform1i           = nullptr;
 PFNGLUNIFORM2FPROC           fn_glUniform2f           = nullptr;
 PFNGLUNIFORM3FPROC           fn_glUniform3f           = nullptr;
 PFNGLUNIFORM4FPROC           fn_glUniform4f           = nullptr;
+PFNGLUNIFORM4FVPROC          fn_glUniform4fv          = nullptr;
 PFNGLUSEPROGRAMPROC          fn_glUseProgram          = nullptr;
 
 namespace fn::gfx {
@@ -67,6 +68,7 @@ bool gl_loader_init() {
     LOAD(glUniform2f);
     LOAD(glUniform3f);
     LOAD(glUniform4f);
+    LOAD(glUniform4fv);
     LOAD(glUseProgram);
 
     #undef LOAD