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/dag_uniforms.cpp

@@ -0,0 +1,27 @@
+#include "gfx/dag_uniforms.h"
+#include "gfx/gl_loader.h"
+#include <algorithm>
+#include <cstring>
+
+namespace fn::gfx {
+
+static constexpr int MAX_NODES = 16;
+
+void dag_uniforms_apply(const std::vector<DagStep>& pipeline, unsigned int program) {
+    float data[MAX_NODES * 4];
+    std::memset(data, 0, sizeof(data));
+
+    const int n = static_cast<int>(std::min(pipeline.size(), static_cast<size_t>(MAX_NODES)));
+    for (int i = 0; i < n; ++i) {
+        const auto& step = pipeline[static_cast<size_t>(i)];
+        data[i * 4 + 0] = step.params[0];
+        data[i * 4 + 1] = step.params[1];
+        data[i * 4 + 2] = step.params[2];
+        data[i * 4 + 3] = step.params[3];
+    }
+
+    GLint loc = glGetUniformLocation(program, "u_params");
+    if (loc >= 0) glUniform4fv(loc, MAX_NODES, data);
+}
+
+} // namespace fn::gfx