feat: Lua table DSL for scene description

[mtao]

Summary

• Adds a Lua-based scene description system using sol2 bindings (new lua meson option, default off)
• Scene scripts return declarative tables that C++ parses into the existing scene graph — combining programmability (loops, functions, require()) with clean declarative syntax
• Includes art-lua CLI runner, 4 example scenes, and 13 test cases (37 assertions)
• Adds PBRT <-> Lua converter tools for importing real PBRT scene files

New Components

Component	Files
Lua bindings	src/lua/bind_{geometry,scene,camera,image,transform}.cpp, src/lua/bindings.cpp, src/lua/bind_internal.hpp
Scene loader	src/lua/scene_loader.cpp, include/art/lua/scene_loader.hpp
Scene serializer	src/lua/scene_serializer.cpp, include/art/lua/scene_serializer.hpp
CLI runner	src/art_lua.cpp — art-lua scene.lua [--output path] [--width N] [--height N] [--accel bvh|linear]
Public API	include/art/lua/bindings.hpp — load_bindings()
Example scenes	scenes/{sphere,transforms,cornell_box,mesh_demo}.lua
Tests	tests/test_scene_loader.cpp — 13 Catch2 test cases
Build system	meson_options.txt (lua option), subprojects/sol2.wrap, subprojects/lua.wrap
PBRT tools	tools/pbrt_to_lua.lua, tools/lua_to_pbrt.lua, tools/validate_roundtrip.lua

Scene DSL Format

return {
    render = { width = 800, height = 600, output = "scene.ppm", accelerator = "bvh" },
    camera = { position = {0,0,5}, target = {0,0,0}, up = {0,1,0} },
    scene = {
        { type = "sphere", transform = { translate = {1,0,0}, scale = 2 } },
        { type = "group", children = { { type = "box" }, { type = "disk" } } },
    },
}

PBRT Converter Tools

Three standalone Lua modules under tools/ (enabled via meson -Dtools=true, default on):

• pbrt_to_lua.lua — Parses PBRT v3/v4 scene files and converts to ART Lua table DSL. Handles all major directives (LookAt, Camera, Film, Shape, Transform, ObjectBegin/End, Include/Import). Supports gzipped Include files (.pbrt.gz). Unsupported features (materials, lights, textures) are consumed gracefully and emitted as TODO comments. Can be used as a module (require("pbrt_to_lua")) or CLI script.

• lua_to_pbrt.lua — Reverse converter from ART Lua scene tables to PBRT v3 format. Supports all ART geometry types with appropriate PBRT mappings.

• validate_roundtrip.lua — Batch validator for testing PBRT->Lua conversion across scene file directories. Supports --max-size, --roundtrip, --verbose, --stop-on-error, --pbrt=<path> flags.

Validation Results

Full test corpus: 386 PBRT files (v3 + v4, under 5MB — files over 5MB are geometry fragments, not standalone scenes).

Validation	Pass	Fail	Skipped
PBRT -> Lua syntax	386	0	0
pbrt --format verify	370	0	16

The 16 skipped files are curve-heavy scenes (hair, bunny-fur) where placeholder spheres generate >50MB PBRT output. All 370 verified files produce PBRT output that the official pbrt-v4 binary accepts without error.

Build & Test

git clone --depth=1 git@github.com:mtao/quiver.git subprojects/quiver  # required for wrap-redirect
meson setup build-debug --buildtype=debug -Dlua=true
meson compile -C build-debug
meson test -C build-debug -v

Future Work

• PLY mesh loading via quiver I/O (currently emits placeholder sphere)
• Matrix transform support in ART's parse_transform() (preserved in Lua output)
• Material / light / texture support