feat(shaders_lab): uniform annotations → auto-generated ImGui controls

- cpp/functions/gfx/uniform_parser: regex-based parser of @slider/@color/@toggle/@xy annotations (+ inline tests)
- cpp/functions/gfx/uniform_panel: ImGui widgets + value store + glUniform* apply
- shader_canvas: optional uniforms callback invoked per-frame
- gl_loader: +glUniform1i/3f/4f
- seed plasma: demo uniforms u_speed + u_color
- rebuild Windows .exe

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

cpp/functions/gfx/uniform_panel.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <array>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "gfx/uniform_parser.h"
+
+namespace fn::gfx {
+
+struct UniformStore {
+    std::unordered_map<std::string, std::array<float, 4>> values;
+};
+
+// Mantiene el store sincronizado con los descriptores actuales.
+void uniforms_sync(UniformStore& store, const std::vector<UniformDescriptor>& descs);
+
+// Renderiza widgets ImGui en el panel actual (llamar entre Begin/End).
+void uniforms_panel(UniformStore& store, const std::vector<UniformDescriptor>& descs);
+
+// Llama glUniform* sobre el programa GL activo (el caller debe haber llamado glUseProgram).
+void uniforms_apply(const UniformStore& store, const std::vector<UniformDescriptor>& descs, unsigned int program);
+
+} // namespace fn::gfx