Refactor rendering backend to explicit capability composition

[Copilot]

The backend API still relied on inherited capability chains and a monolithic Haru implementation, which kept line/text/shape/page/image concerns more coupled than intended. This change moves the backend contract to explicit capability accessors and reshapes the Haru backend into composed internal capability components while preserving existing rendering behavior.

• Backend contract

  • Make capability interfaces standalone and chain-free.
  • Change IDocraftRenderingBackend from an inheritance aggregator to a root contract exposing explicit capability accessors:
    • line_rendering() / edit_line_rendering()
    • text_rendering() / edit_text_rendering()
    • shape_rendering() / edit_shape_rendering()
    • image_rendering() / edit_image_rendering()
    • page_rendering() / edit_page_rendering()

• Haru backend composition

  • Split Haru backend responsibilities into focused internal components for text, line, shape, image, and page capabilities.
  • Keep DocraftHaruBackend as the single integration point and route capability access through composed objects instead of implementing every primitive directly on the root class.

• Backend wiring

  • Update backend cache initialization to bind capabilities through root accessors instead of cross-casting the root backend to sub-interfaces.
  • Keep document/context integration unchanged at the orchestration level while making capability retrieval explicit internally.

• Renderer and painter migration

  • Update affected renderers/painters to request the exact capabilities they use instead of relying on inherited methods leaking across interfaces.
  • Separate shape-state usage from line drawing where the old interface chain previously hid that boundary.

• Coverage and docs

  • Add accessor wiring coverage for const vs mutable capability access and shared underlying component identity.
  • Refresh backend API/contributor docs to describe the composition model and explicit accessor pattern.

const auto *line = backend.line_rendering();
auto *shape = backend.edit_shape_rendering();
auto *page = backend.edit_page_rendering();