Add $vpr/$vpt/$vpd viewport special variables (v3.5 deferred item)

[particlesector]

Summary

• Picks up the first of the two items explicitly deferred at the end of the v3.5 pass: $vpr/$vpt/$vpd viewport special variables (the second — making cube/sphere/translate/etc. ordinary identifiers instead of reserved keywords — is a genuine grammar/semantics refactor and stays deferred; see docs/roadmap.md).
• Interpreter gains a setViewport() setter and $vpr/$vpt/$vpd lookups, mirroring the existing $preview/$t VarRef-fallback pattern, with defaults matching OpenSCAD's own ($vpr=[55,0,25], $vpt=[0,0,0], $vpd=140) so headless/test evaluation returns real numbers instead of undef.
• MeshBuilder::requestBuild() now takes a ViewportState snapshot, threaded through a pre-populated Interpreter into CsgEvaluator's existing evaluate(result, interp) overload — the same wiring pattern already used for fileTable.
• Application::currentViewport() derives that snapshot from the render camera's pitch/yaw/target/distance at each requestBuild() call site (7 sites updated).

Test plan

• Built a standalone copy of the project's full non-GPU test suite (the same source set as the chiselcad_tests CMake target — Lexer/Parser/Interpreter/SourceLoader/CsgEvaluator/importers) using apt-installed glm/spdlog/Catch2 in place of vcpkg (no Vulkan SDK needed for this target). 2877 assertions across 533 test cases pass, including new tests for the $vpr/$vpt/$vpd defaults, setViewport(), and user-assignment shadowing.
• Syntax-checked MeshBuilder.cpp against stubbed manifold/VMA headers — caught and fixed a real GCC quirk (a nested aggregate used as a default argument of its own enclosing class's member function).
• Application.cpp's diff (7 mechanical call-site argument additions plus one small accessor) could not be compiled in this sandbox — the full GLFW/ImGui/Vulkan stack isn't available here — so it was reviewed by hand against Camera's verified public API instead of compiled. CI's full Windows/Linux builds will cover it.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MEXUhKnGAnmaLJ4R8PEg7r

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Generated by Claude Code

[particlesector]

Reviewed. The wiring checks out: MeshBuilder::buildOne's manual Interpreter/loadAssignments/loadFunctions/evaluate(ast, interp) sequence is an exact match for what CsgEvaluator::evaluate(ast)'s convenience overload already does internally, so setViewport() is the only added behavior — no regression risk there. All 7 requestBuild() call sites in Application.cpp are updated, Camera::pitch()/yaw()/target()/distance() exist with the signatures currentViewport() assumes, and the $vpr/$vpt/$vpd VarRef fallback correctly matches the existing $preview/$t pattern (env lookup first, so user assignments still shadow). No issues found.

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Generated by Claude Code