feat(shaders_lab): scaffold C++ app with GLSL live-reload canvas

- cpp/functions/gfx: gl_shader, gl_framebuffer, fullscreen_quad, shader_canvas
- cpp/apps/shaders_lab: main + 3 seed shaders (plasma, circle, checker)
- ImGui docking layout: Code | Canvas | Controls

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

cpp/functions/gfx/gl_shader.cpp

@@ -0,0 +1,119 @@
+#define GL_GLEXT_PROTOTYPES
+#include <GL/gl.h>
+#include <GL/glext.h>
+
+#include "gfx/gl_shader.h"
+
+#include <cstdio>
+#include <cstring>
+#include <regex>
+#include <string>
+
+namespace fn::gfx {
+
+static const char* k_vert_src = R"glsl(
+#version 330 core
+const vec2 verts[6] = vec2[](
+    vec2(-1,-1), vec2(1,-1), vec2(-1,1),
+    vec2(1,-1),  vec2(1,1),  vec2(-1,1)
+);
+void main() { gl_Position = vec4(verts[gl_VertexID], 0.0, 1.0); }
+)glsl";
+
+static const char* k_frag_preamble =
+    "#version 330 core\n"
+    "out vec4 fragColor;\n"
+    "uniform vec2 u_resolution;\n"
+    "uniform float u_time;\n"
+    "uniform vec2 u_mouse;\n";
+
+static int parse_err_line(const char* log) {
+    // Try "ERROR: 0:<line>:" format
+    std::regex re1(R"(ERROR:\s*\d+:(\d+):)");
+    // Try "0(<line>)" format
+    std::regex re2(R"(\d+\((\d+)\))");
+    std::cmatch m;
+    if (std::regex_search(log, m, re1)) {
+        return std::stoi(m[1].str());
+    }
+    if (std::regex_search(log, m, re2)) {
+        return std::stoi(m[1].str());
+    }
+    return -1;
+}
+
+static unsigned int compile_shader(GLenum type, const char** srcs, int count, std::string& out_err, int& out_line) {
+    unsigned int s = glCreateShader(type);
+    glShaderSource(s, count, srcs, nullptr);
+    glCompileShader(s);
+    GLint ok = 0;
+    glGetShaderiv(s, GL_COMPILE_STATUS, &ok);
+    if (!ok) {
+        GLint len = 0;
+        glGetShaderiv(s, GL_INFO_LOG_LENGTH, &len);
+        out_err.resize(static_cast<size_t>(len));
+        glGetShaderInfoLog(s, len, nullptr, &out_err[0]);
+        out_line = parse_err_line(out_err.c_str());
+        glDeleteShader(s);
+        return 0;
+    }
+    return s;
+}
+
+CompileResult compile_fragment(const std::string& user_fragment_src) {
+    CompileResult result;
+
+    // Vertex shader (no preamble needed — it's a standalone complete shader)
+    std::string vert_err;
+    int vert_line = -1;
+    unsigned int vert = compile_shader(GL_VERTEX_SHADER, &k_vert_src, 1, vert_err, vert_line);
+    if (!vert) {
+        result.err_msg = "vertex: " + vert_err;
+        result.err_line = vert_line;
+        return result;
+    }
+
+    // Fragment shader — prepend preamble, then user body
+    const char* frag_srcs[2] = { k_frag_preamble, user_fragment_src.c_str() };
+    std::string frag_err;
+    int frag_line = -1;
+    unsigned int frag = compile_shader(GL_FRAGMENT_SHADER, frag_srcs, 2, frag_err, frag_line);
+    if (!frag) {
+        glDeleteShader(vert);
+        result.err_msg = frag_err;
+        // Adjust line: subtract preamble lines (4 lines)
+        if (frag_line > 4) result.err_line = frag_line - 4;
+        else result.err_line = frag_line;
+        return result;
+    }
+
+    // Link
+    unsigned int prog = glCreateProgram();
+    glAttachShader(prog, vert);
+    glAttachShader(prog, frag);
+    glLinkProgram(prog);
+    glDeleteShader(vert);
+    glDeleteShader(frag);
+
+    GLint ok = 0;
+    glGetProgramiv(prog, GL_LINK_STATUS, &ok);
+    if (!ok) {
+        GLint len = 0;
+        glGetProgramiv(prog, GL_INFO_LOG_LENGTH, &len);
+        result.err_msg.resize(static_cast<size_t>(len));
+        glGetProgramInfoLog(prog, len, nullptr, &result.err_msg[0]);
+        result.err_line = parse_err_line(result.err_msg.c_str());
+        glDeleteProgram(prog);
+        return result;
+    }
+
+    result.program = prog;
+    result.ok = true;
+    return result;
+}
+
+void delete_program(unsigned int program) {
+    if (program) glDeleteProgram(program);
+}
+
+} // namespace fn::gfx