Add generic shell-texturing fur shader and renderer

FurShell.shader renders N normal-extruded shells with per-strand
alpha-cutout for hair/fur/grass on any mesh (needs only normals/UVs).
Parameters cover length, density, thickness, taper, tip fill, curvature,
per-strand direction randomness, wind sway and silhouette strand stretch.

FurShellRenderer drives every parameter via MaterialPropertyBlock and
draws the mesh shellCount times with RenderMeshInstanced from
beginCameraRendering (works in edit, game and screenshot cameras).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Components/FurShellRenderer.cs.meta

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