Vulkan RT: smooth vertex-normal shading and normal-pool offset fix

[brianmk]

Coin-side counterpart of brianmk/FreeCAD-vulkan#30.

Changes

• Normal pool stores each triangle's three vertex normals; the ray-query tracer
  barycentric-interpolates them (SBT closest-hit uses the averaged vertex normal)
  so curved solids shade smoothly.
• Fix ensureNormalPoolCapacity(): growing the pool reset normalPoolUsed to 0,
  so every command that triggered a grow got offset 0 and read the last-written
  geometry's normals (the cylinder cap picked up the sphere's radial normals and
  shaded as a per-triangle fan). Preserve existing records across a grow.
• Fall back to the viewport clear color for the RT sky when no gradient is set.
• Skip the overlay depth clear for full-frame overlays (selection / preselect)
  so highlights are not drawn over occluding geometry.
• Re-key a cached geometry only when the geometry matches, so an overlay command
  reusing a traced command's draw-list slot no longer restarts path tracing.
• Grant contents: write to the documentation workflow so
  peaceiris/actions-gh-pages can publish the gh-pages branch (the default
  GITHUB_TOKEN is read-only, which was failing the default-branch run with a
  403).

Verification

• Pool offsets before: box=0, cylinder=0, sphere=0. After: box=0, cylinder=48,
  sphere=4368.
• Path-traced render: cylinder cap is a smooth uniform disk (fan gone).
• Coin, FreeCADGui, FreeCADMain build green; changed TUs compile clean
  under clang++ -Werror.