Refactor rendering backend to explicit capability composition

doc/project-doc/contributors/components/backend-integration.md

@@ -17,17 +17,21 @@ Important implication:
 
 ## 2. Interface contract you must implement
 
-A backend must implement `IDocraftRenderingBackend`, which aggregates:
+A backend must implement `IDocraftRenderingBackend`, which exposes capability
+accessors for:
 
 - text primitives,
-- line/shape primitives,
+- line primitives,
+- shape primitives,
 - image primitives,
 - page management,
 - save/output extension,
 - metadata application,
 - font registration and font selection hooks.
 
-In practice you implement one concrete class inheriting `IDocraftRenderingBackend`.
+In practice you implement one concrete class inheriting
+`IDocraftRenderingBackend` and return the capability objects from the root via
+`<capability>() const` and `edit_<capability>()`.
 
 ## 3. External integration (recommended path)
 
@@ -64,7 +68,8 @@ void render_with_external_backend(const std::string &craft_path) {
 Typical steps for a backend added directly in this repository:
 
 1. Add backend class, for example `docraft::backend::svg::DocraftSvgBackend`.
-2. Implement all methods of `IDocraftRenderingBackend`.
+2. Implement all root methods of `IDocraftRenderingBackend` and wire each
+   capability accessor to the appropriate capability object.
 3. Add new source/header files to `docraft/CMakeLists.txt`.
 4. Add tests in `docraft/test/backend/` and/or rendering smoke tests.
 5. Choose selection strategy:

doc/project-doc/contributors/components/backend.md

@@ -4,9 +4,10 @@ The backend layer is the portability boundary for output targets.
 
 ## 1. Contract hierarchy
 
-`IDocraftRenderingBackend` aggregates these contracts:
+`IDocraftRenderingBackend` exposes these contracts via explicit accessors:
 
 - `IDocraftTextRenderingBackend`
+- `IDocraftLineRenderingBackend`
 - `IDocraftShapeRenderingBackend`
 - `IDocraftImageRenderingBackend`
 - `IDocraftPageRenderingBackend`
@@ -18,17 +19,22 @@ Plus lifecycle methods:
 - font registration/selection helpers
 - metadata application
 
-This design lets renderers/painters consume only the primitives they need.
+Capability interfaces are standalone (no inheritance chain between them), and
+the root backend provides `<capability>() const` plus `edit_<capability>()`
+accessors so renderers/painters can consume only the primitives they need.
 
 ## 2. Concrete implementation: Haru backend
 
-`DocraftHaruBackend` implements all contracts over libharu.
+`DocraftHaruBackend` remains the single concrete backend entry point, but it
+composes smaller libharu-backed capability objects internally.
 
 Responsibilities include:
 
 - managing document/page handles,
 - text drawing and measurement,
-- line/shape/image drawing,
+- line drawing,
+- shape drawing and graphics state,
+- image drawing,
 - page navigation and page format,
 - metadata mapping to PDF info fields,
 - save to `.pdf`.

doc/source/api/backend.rst

@@ -8,7 +8,9 @@ output.
 IDocraftRenderingBackend
 ------------------------
 
-Aggregated interface inheriting all sub-backend interfaces.
+Root backend interface exposing lifecycle/font operations plus explicit
+capability accessors such as ``line_rendering()`` / ``edit_line_rendering()``
+and the corresponding text, shape, image, and page accessors.
 
 .. doxygenclass:: docraft::backend::IDocraftRenderingBackend
    :project: docraft
@@ -17,6 +19,8 @@ Aggregated interface inheriting all sub-backend interfaces.
 Sub-backend Interfaces
 ----------------------
 
+Capability interfaces are standalone and chain-free.
+
 IDocraftTextRenderingBackend
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -58,9 +62,9 @@ Concrete Backends
 DocraftHaruBackend
 ^^^^^^^^^^^^^^^^^^
 
-PDF backend implementation using libharu.
+PDF backend implementation using libharu, composed from capability-focused
+internal objects behind the root backend.
 
 .. doxygenclass:: docraft::backend::pdf::DocraftHaruBackend
    :project: docraft
    :members:
-

docraft/include/docraft/backend/docraft_rendering_backend.h

@@ -19,6 +19,7 @@
 #include "docraft/docraft_lib.h"
 #include <string>
 
+#include "docraft/backend/docraft_line_rendering_backend.h"
 #include "docraft/backend/docraft_image_rendering_backend.h"
 #include "docraft/backend/docraft_page_rendering_backend.h"
 #include "docraft/backend/docraft_shape_rendering_backend.h"
@@ -32,15 +33,52 @@ namespace docraft::backend {
     /**
      * @brief Aggregated rendering backend interface.
      */
-    class DOCRAFT_LIB IDocraftRenderingBackend : public IDocraftTextRenderingBackend,
-                                     public IDocraftShapeRenderingBackend,
-                                     public IDocraftImageRenderingBackend,
-                                     public IDocraftPageRenderingBackend {
+    class DOCRAFT_LIB IDocraftRenderingBackend {
     public:
         /**
          * @brief Virtual destructor.
          */
-        ~IDocraftRenderingBackend() override = default;
+        virtual ~IDocraftRenderingBackend() = default;
+        /**
+         * @brief Returns the line rendering capability, if available.
+         */
+        [[nodiscard]] virtual const IDocraftLineRenderingBackend *line_rendering() const = 0;
+        /**
+         * @brief Returns the editable line rendering capability, if available.
+         */
+        [[nodiscard]] virtual IDocraftLineRenderingBackend *edit_line_rendering() = 0;
+        /**
+         * @brief Returns the text rendering capability, if available.
+         */
+        [[nodiscard]] virtual const IDocraftTextRenderingBackend *text_rendering() const = 0;
+        /**
+         * @brief Returns the editable text rendering capability, if available.
+         */
+        [[nodiscard]] virtual IDocraftTextRenderingBackend *edit_text_rendering() = 0;
+        /**
+         * @brief Returns the shape rendering capability, if available.
+         */
+        [[nodiscard]] virtual const IDocraftShapeRenderingBackend *shape_rendering() const = 0;
+        /**
+         * @brief Returns the editable shape rendering capability, if available.
+         */
+        [[nodiscard]] virtual IDocraftShapeRenderingBackend *edit_shape_rendering() = 0;
+        /**
+         * @brief Returns the image rendering capability, if available.
+         */
+        [[nodiscard]] virtual const IDocraftImageRenderingBackend *image_rendering() const = 0;
+        /**
+         * @brief Returns the editable image rendering capability, if available.
+         */
+        [[nodiscard]] virtual IDocraftImageRenderingBackend *edit_image_rendering() = 0;
+        /**
+         * @brief Returns the page rendering capability, if available.
+         */
+        [[nodiscard]] virtual const IDocraftPageRenderingBackend *page_rendering() const = 0;
+        /**
+         * @brief Returns the editable page rendering capability, if available.
+         */
+        [[nodiscard]] virtual IDocraftPageRenderingBackend *edit_page_rendering() = 0;
         /**
          * @brief Saves the document to a file path.
          * @param path Output file path.

docraft/include/docraft/backend/docraft_shape_rendering_backend.h

@@ -20,19 +20,18 @@
 
 #include <vector>
 
-#include "docraft/backend/docraft_line_rendering_backend.h"
 #include "docraft/model/docraft_position.h"
 
 namespace docraft::backend {
     /**
      * @brief Interface for shape rendering backends used by Docraft.
      */
-    class DOCRAFT_LIB IDocraftShapeRenderingBackend : public virtual IDocraftLineRenderingBackend {
+    class DOCRAFT_LIB IDocraftShapeRenderingBackend {
     public:
         /**
          * @brief Virtual destructor.
          */
-        ~IDocraftShapeRenderingBackend() override = default;
+        virtual ~IDocraftShapeRenderingBackend() = default;
         /**
          * @brief Saves the current graphics state.
          */

docraft/include/docraft/backend/docraft_text_rendering_backend.h

@@ -19,18 +19,16 @@
 #include "docraft/docraft_lib.h"
 #include <string>
 
-#include "docraft/backend/docraft_line_rendering_backend.h"
-
 namespace docraft::backend {
     /**
      * @brief Interface for text rendering backends used by Docraft.
      */
-    class DOCRAFT_LIB IDocraftTextRenderingBackend : public virtual IDocraftLineRenderingBackend {
+    class DOCRAFT_LIB IDocraftTextRenderingBackend {
     public:
         /**
          * @brief Virtual destructor.
          */
-        ~IDocraftTextRenderingBackend() override = default;
+        virtual ~IDocraftTextRenderingBackend() = default;
         /**
          * @brief Initializes the text rendering context. Must be called before any text drawing operations.
          */