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/apps/shaders_lab/seed_shaders.h

@@ -4,24 +4,14 @@
 // compile_fragment() prepends those automatically.
 
 static const char* PLASMA = R"glsl(
+uniform float u_speed;   // @slider min=0.1 max=5 default=1
+uniform vec3  u_color;   // @color default=0.5,0.2,0.8
+
 void main() {
     vec2 uv = gl_FragCoord.xy / u_resolution;
-    float t = u_time * 0.5;
-
-    float v1 = sin(uv.x * 10.0 + t);
-    float v2 = sin(uv.y * 10.0 + t * 1.3);
-    float v3 = sin((uv.x + uv.y) * 10.0 + t * 0.7);
-    float v4 = sin(length(uv - 0.5) * 20.0 - t * 2.0);
-
-    float v = (v1 + v2 + v3 + v4) * 0.25;
-
-    vec3 col = vec3(
-        sin(v * 3.14159 + 0.0) * 0.5 + 0.5,
-        sin(v * 3.14159 + 2.094) * 0.5 + 0.5,
-        sin(v * 3.14159 + 4.188) * 0.5 + 0.5
-    );
-
-    fragColor = vec4(col, 1.0);
+    float t = u_time * u_speed;
+    vec3 c = u_color * (0.5 + 0.5 * cos(t + uv.xyx + vec3(0.0, 2.0, 4.0)));
+    fragColor = vec4(c, 1.0);
 }
 )glsl";