Use Perastage SVG symbols for 2D layout fixtures with mesh fallback

[PeramatoG]

Motivation

• Make layout editor 2D view elements behave like legends by reusing Perastage/GDTF SVG symbols when available so identical models instantiate the same 2D symbol; keep the existing mesh/cube capture flow as a fallback when no SVG exists or is invalid.

Description

• Add a small helper perastage_svg_symbol_builder (viewer3d/render/perastage_svg_symbol_builder.{h,cpp}) that loads Perastage SVG from a GDTF, converts fills and strokes to CommandBuffer commands, applies model offsets and computes symbol bounds.
• Update the fixture capture path in viewer3d/render/opaque_fixture_pass.cpp to try building an SVG-backed SymbolDefinition first and return it to the bottom-symbol cache when successful.
• Register the new source file in viewer3d/CMakeLists.txt to keep explicit source ownership and module boundaries intact.

Testing

• Ran tests/check_perastage_tree_modules.sh and it passed.
• Ran tests/check_no_configmanager_get_in_gui.sh and it passed.

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Codex Task