Add PictureFrameBuilder procedural picture frame component

Editor-tunable frame mesh: sliced scaling (constant profile), 4 material
submeshes (picture/interior/frame/back), padding, frame thickness/depth,
rounded outer edges (quarter-cylinder strips + eighth-sphere corner caps).
Frame Inner Depth sets the interior plane; Picture Depth is relative to it
(positive = recessed, negative = raised pedestal) with auto perimeter walls.
Meshes are per-instance, DontSave, rebuilt in OnEnable/OnValidate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Components/PictureFrameBuilder.cs.meta

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