feat(cpp/core): primitivas UI estilo Mantine

Anade 9 primitivas reutilizables al registry C++ que replican el comportamiento
de los componentes correspondientes de @fn_library / Mantine v9, todas
estilizadas con tokens_cpp_core (colores Mantine dark + indigo):

- button_cpp_core         (component, pure)  variantes primary/secondary/subtle/danger + sm/md/lg
- icon_button_cpp_core    (component, pure)  cuadrado 28x28 con glyph centrado + tooltip
- toolbar_cpp_core        (component, pure)  grupo horizontal de acciones con separadores
- modal_dialog_cpp_core   (component, pure)  popup modal centrada + close con Escape
- text_input_cpp_core     (component, impure) InputText con label muted + placeholder
- select_cpp_core         (component, impure) dropdown con label + opcion '(none)' opcional
- toast_cpp_core          (component, impure) notificaciones efimeras + inbox con badge
- tree_view_cpp_core      (component, impure) jerarquia low-level con tree_node_clicked helper
- process_runner_cpp_core (component, impure) tarea en std::thread + spinner inline

Cada primitiva tiene su .md con frontmatter completo (params/output) y se
indexa via fn index. Son la base del primitives_gallery y de cualquier
app fn_ui futura.

cpp/functions/core/button.cpp

@@ -0,0 +1,74 @@
+#include "core/button.h"
+#include "core/tokens.h"
+#include <imgui.h>
+
+namespace fn_ui {
+
+namespace {
+
+struct VariantColors {
+    ImVec4 bg;
+    ImVec4 bg_hover;
+    ImVec4 bg_active;
+    ImVec4 text;
+    ImVec4 border;
+    float border_size; // 0 = sin borde
+};
+
+VariantColors colors_for(ButtonVariant v) {
+    using namespace fn_tokens::colors;
+    switch (v) {
+        case ButtonVariant::Primary:
+            return {primary,       primary_hover, primary,       text,       primary,       0.0f};
+        case ButtonVariant::Secondary:
+            return {surface,       surface_hover, surface,       text,       border,        1.0f};
+        case ButtonVariant::Subtle:
+            return {ImVec4(0,0,0,0), surface_hover, surface,     primary,    ImVec4(0,0,0,0),0.0f};
+        case ButtonVariant::Danger:
+            return {error,         error,         error,         text,       error,         0.0f};
+    }
+    return {surface, surface_hover, surface, text, border, 1.0f};
+}
+
+ImVec2 padding_for(ButtonSize s) {
+    using namespace fn_tokens::spacing;
+    switch (s) {
+        case ButtonSize::Sm: return ImVec2(md, xs + 2.0f);
+        case ButtonSize::Md: return ImVec2(lg, sm);
+        case ButtonSize::Lg: return ImVec2(xl, md);
+    }
+    return ImVec2(lg, sm);
+}
+
+float font_scale_for(ButtonSize s) {
+    return (s == ButtonSize::Lg) ? 1.1f : 1.0f;
+}
+
+} // namespace
+
+bool button(const char* label, ButtonVariant variant, ButtonSize size) {
+    using namespace fn_tokens;
+
+    const VariantColors c = colors_for(variant);
+    const ImVec2 pad = padding_for(size);
+    const float fs = font_scale_for(size);
+
+    ImGui::PushStyleColor(ImGuiCol_Button,        c.bg);
+    ImGui::PushStyleColor(ImGuiCol_ButtonHovered, c.bg_hover);
+    ImGui::PushStyleColor(ImGuiCol_ButtonActive,  c.bg_active);
+    ImGui::PushStyleColor(ImGuiCol_Text,          c.text);
+    ImGui::PushStyleColor(ImGuiCol_Border,        c.border);
+    ImGui::PushStyleVar(ImGuiStyleVar_FrameRounding,  radius::sm);
+    ImGui::PushStyleVar(ImGuiStyleVar_FrameBorderSize, c.border_size);
+    ImGui::PushStyleVar(ImGuiStyleVar_FramePadding,    pad);
+
+    if (fs != 1.0f) ImGui::SetWindowFontScale(fs);
+    const bool clicked = ImGui::Button(label);
+    if (fs != 1.0f) ImGui::SetWindowFontScale(1.0f);
+
+    ImGui::PopStyleVar(3);
+    ImGui::PopStyleColor(5);
+    return clicked;
+}
+
+} // namespace fn_ui