Add deterministic procedural mesh toolkit (procgen) with primitives, spline extrusion, lathe, segments, SDF/volume & shell demo

[ercmine]

Motivation

• Implement Statement 17 by adding an engine-owned, deterministic procedural mesh foundation to produce reusable CPU-side geometry for rooms, bugs, items, props, VFX, and tools.
• Provide a compact, renderer-agnostic API that feeds the existing CpuMeshData/renderer upload path while remaining deterministic and testable.
• Deliver practical generation modes (primitives, spline extrusion, lathing, segment assembly, voxel-like scalar fields, and SDF composition + extraction) rather than a full DCC/editor system.

Description

• Added a new module engine/procgen exposing procgen.hpp/procgen.cpp with ProcVertex, generation entry points (primitives, extrude_profile_along_path, lathe_profile, assemble_segments), scalar field and SDF composition helpers, marching-tetrahedra extraction, and mesh post-process/validation utilities.
• Implemented core primitives (make_quad, make_plane, make_box, make_uv_sphere, make_cylinder, make_cone, make_torus, make_capsule), spline utilities (Catmull‑Rom sampling + frame derivation), extrusion and lathe generation, deterministic segment assembly using core::Seed/core::Random splits, ScalarField storage, and SDF primitives/composition functions.
• Integrated procgen into the build (CMakeLists.txt) and added tests/procgen/procgen_tests.cpp exercising primitive/spline/lathe/segment/SDF/extraction determinism and validation, plus updated the runtime shell (engine/shell/main.cpp) to upload and render representative generated meshes via existing create_mesh_buffer/create_index_buffer APIs.
• Added documentation docs/procgen.md and updated docs/architecture.md to describe organization, determinism, generation conventions, integration, and known limits.

Testing

• Attempting a full CMake configure/build with cmake --preset linux-debug failed in this environment due to missing third-party dependencies (SDL3/bgfx), so the full CMake run could not be completed here.
• Performed syntax/compile checks with g++ -std=c++20 -I. -fsyntax-only engine/procgen/procgen.cpp tests/procgen/procgen_tests.cpp and g++ -std=c++20 -I. -fsyntax-only engine/shell/main.cpp, both of which succeeded.
• Built and ran a standalone test binary with g++ -std=c++20 -I. tests/procgen/procgen_tests.cpp engine/procgen/procgen.cpp engine/render/buffer_types.cpp engine/core/math.cpp engine/core/transform.cpp engine/core/random.cpp engine/core/hash.cpp -o /tmp/procgen_tests && /tmp/procgen_tests, and the procgen test ran successfully (assertions passed).
• Registered render_procgen_tests in CMake and added CTest entry unit.procgen.mesh_toolkit so the suite will run under CI once external dependencies are present.

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Codex Task