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/shader_canvas.cpp

@@ -1,6 +1,7 @@
 #include "gfx/gl_loader.h"
 #include "gfx/shader_canvas.h"
 #include "imgui.h"
+#include <functional>
 
 namespace fn::gfx {
 
@@ -17,7 +18,8 @@ void canvas_set_program(ShaderCanvas& c, unsigned int program) {
     c.program = program;
 }
 
-void canvas_render(ShaderCanvas& c, float time_seconds) {
+void canvas_render(ShaderCanvas& c, float time_seconds,
+                   const std::function<void(unsigned int program)>& uniforms_fn) {
     ImVec2 avail = ImGui::GetContentRegionAvail();
     int w = static_cast<int>(avail.x);
     int h = static_cast<int>(avail.y);
@@ -56,6 +58,8 @@ void canvas_render(ShaderCanvas& c, float time_seconds) {
             glUniform2f(loc_mouse, mx, my);
         }
 
+        if (uniforms_fn) uniforms_fn(c.program);
+
         quad_draw(c.quad);
         glUseProgram(0);
     }