feat: add gfx pipeline abstraction with render targets

Add cbt/gfx layer (pipeline + render_target) inspired by Sokol but
C++-friendly with RAII and pipeline_desc. Supports building full
graphics pipelines (VS/IA/raster/PS/OM), render-to-texture (FBO),
and post-processing steps (scene -> RT -> fullscreen quad with
sampler + vignette).

- Depth/state/viewport/texture cleanup for reliable scene switching.
- Updated cube/sphere to demonstrate (sphere uses RT+post).
- Vulkan backend easy via PIMPL in impl (same public API).
- Fixed depth test, viewport restore, and state leakage on switch.

Followed coding conventions (snake_case, trailing returns, east const,
include order, no ; after ns/class, etc.).

scenes/cube.cpp

@@ -1,8 +1,8 @@
 #include <array>
+#include <span>
 
 #include "glad/glad.h"
 #include "glm/gtc/matrix_transform.hpp"
-#include "glm/gtc/type_ptr.hpp"
 
 #include "scenes/cube.hpp"
 
@@ -13,17 +13,17 @@ cube::cube() {
 }
 
 auto cube::init() -> bool {
-    if (!build_shader()) {
+    if (!build_pipeline()) {
         return false;
     }
-    build_mesh();
-    glEnable(GL_DEPTH_TEST);
     return true;
 }
 
 auto cube::update(float) -> void {}
 
 auto cube::render(int width, int height) -> void {
+    glViewport(0, 0, width, height);
+
     auto now = std::chrono::steady_clock::now();
     auto elapsed = std::chrono::duration<float>(now - m_start).count();
 
@@ -35,18 +35,33 @@ auto cube::render(int width, int height) -> void {
     auto view = glm::translate(glm::mat4{1.0f}, glm::vec3{0.0f, 0.0f, -3.0f});
     auto model = glm::rotate(glm::mat4{1.0f}, elapsed, glm::vec3{1.0f, 0.5f, 0.3f});
 
-    m_prog.use();
-    glUniformMatrix4fv(m_loc_proj, 1, GL_FALSE, glm::value_ptr(proj));
-    glUniformMatrix4fv(m_loc_view, 1, GL_FALSE, glm::value_ptr(view));
-    glUniformMatrix4fv(m_loc_model, 1, GL_FALSE, glm::value_ptr(model));
-
-    m_vao.bind();
-    glDrawArrays(GL_TRIANGLES, 0, 36);
-    m_vao.unbind();
-    m_prog.unuse();
+    m_pipeline.draw(model, view, proj);
 }
 
-auto cube::build_mesh() -> void {
+auto cube::build_pipeline() -> bool {
+    char const* vert_src = R"glsl(
+        #version 410 core
+        layout(location = 0) in vec3 a_pos;
+        layout(location = 1) in vec3 a_color;
+        uniform mat4 u_model;
+        uniform mat4 u_view;
+        uniform mat4 u_proj;
+        out vec3 v_color;
+        void main() {
+            gl_Position = u_proj * u_view * u_model * vec4(a_pos, 1.0);
+            v_color = a_color;
+        }
+    )glsl";
+
+    char const* frag_src = R"glsl(
+        #version 410 core
+        in vec3 v_color;
+        out vec4 frag_color;
+        void main() {
+            frag_color = vec4(v_color, 1.0);
+        }
+    )glsl";
+
     std::array<float, 36 * 6> vertices = {
         // front face (cyan)
          0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
@@ -97,52 +112,22 @@ auto cube::build_mesh() -> void {
         -0.5f,  0.5f, -0.5f,  1.0f, 0.5f, 0.0f,
     };
 
-    m_vbo.upload(vertices.data(), vertices.size() * sizeof(float));
+    gfx::pipeline_desc desc{
+        .vertex_data = std::as_bytes(std::span{vertices}),
+        .attributes = {
+            {.location = 0, .num_components = 3, .offset = 0},
+            {.location = 1, .num_components = 3, .offset = 12},
+        },
+        .vertex_stride = 24,
+        .vertex_shader_src = vert_src,
+        .fragment_shader_src = frag_src,
+        .depth_test = true,
+        .primitive = gfx::primitive_type::triangles
+        // no index data
+    };
 
-    m_vao.bind();
-    m_vbo.bind();
-    glEnableVertexAttribArray(0);
-    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), nullptr);
-    glEnableVertexAttribArray(1);
-    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float),
-                         reinterpret_cast<void*>(3 * sizeof(float)));
-    m_vbo.unbind();
-    m_vao.unbind();
-}
-
-auto cube::build_shader() -> bool {
-    char const* vert_src = R"glsl(
-        #version 410 core
-        layout(location = 0) in vec3 a_pos;
-        layout(location = 1) in vec3 a_color;
-        uniform mat4 u_model;
-        uniform mat4 u_view;
-        uniform mat4 u_proj;
-        out vec3 v_color;
-        void main() {
-            gl_Position = u_proj * u_view * u_model * vec4(a_pos, 1.0);
-            v_color = a_color;
-        }
-    )glsl";
-
-    char const* frag_src = R"glsl(
-        #version 410 core
-        in vec3 v_color;
-        out vec4 frag_color;
-        void main() {
-            frag_color = vec4(v_color, 1.0);
-        }
-    )glsl";
-
-    if (!m_prog.compile_vertex(vert_src) || !m_prog.compile_fragment(frag_src) || !m_prog.link()) {
-        return false;
-    }
-
-    m_loc_proj = glGetUniformLocation(m_prog.id(), "u_proj");
-    m_loc_view = glGetUniformLocation(m_prog.id(), "u_view");
-    m_loc_model = glGetUniformLocation(m_prog.id(), "u_model");
-
-    return true;
+    m_pipeline = gfx::pipeline{desc};
+    return m_pipeline.valid();
 }
 
 }