feat(pj_base): add VoxelGrid canonical builtin object (SDK 0.10.0)

[facontidavide]

Summary

Adds sdk::VoxelGrid — a new canonical builtin object for dense 3D voxel grids (3D occupancy maps, costmaps, ESDFs, semantic grids). It is the volumetric sibling of OccupancyGrid, reusing PointCloud's PointField channel model.

The struct mirrors foxglove.VoxelGrid's byte layout (depth-major Z-Y-X order + three strides). Because PJ parses messages lazily (the store keeps the original serialized bytes), this fidelity lets a producer expose data as a zero-copy Span<const uint8_t> + BufferAnchor instead of transcoding millions of cells. The per-voxel value is generic (occupancy byte / RGBA / float cost / class id via fields); which voxels are drawn — and any colormap — stays a viewer-side decision (unlike the 2D OccupancyGrid, which fixes -1/0..100).

Why Foxglove's shape and not Rerun's: Foxglove is the only ecosystem with a dedicated VoxelGrid schema; Rerun has no voxel type (it expands to Boxes3D at render time). So this is the canonical storage schema; sparse/instanced GPU expansion is a downstream pj_scene3D renderer concern, not part of the SDK.

Changes

• pj_base/builtin/voxel_grid.hpp — the struct (zero-copy data + BufferAnchor, PointField reuse)
• pj_base/builtin/voxel_grid_codec.{hpp,cpp} — canonical wire codec (hand-rolled protobuf_wire, no protobuf runtime)
• pj_base/proto/pj/VoxelGrid.proto — wire contract (imports Geometry.proto + PointCloud.proto)
• kVoxelGrid = 18 appended to both BuiltinObjectType (builtin_object.hpp) and PJ_builtin_object_type_t (builtin_object_abi.h) — append-only; type 2 stays reserved
• Tests: voxel_grid_codec_test.cpp (round-trip + empty-buffer + schema-name); builtin_object_test enumerations extended
• Docs: builtin_type.md (table row, per-type section, conversion row), proto README, builtin-count bumps + corrected 3-source version mechanics in CLAUDE.md

Versioning

Bumps to 0.10.0 — a MINOR (backward-compatible addition: tail-appended enum value + two new exported symbols). abi/baseline.abi is unchanged (it only moves on a MAJOR; abidiff should show additions only). All three version sources (conanfile.py, CMakeLists.txt, recipe.yaml) are aligned so the conda-release guard passes — recipe.yaml had drifted to 0.8.1, which is why the conda/pixi 0.9.0 publish failed.

Plugins/PJ4 adopt plotjuggler_sdk/[>=0.10.0 <1.0.0] once released.

Test plan

• ./build.sh --debug && ./test.sh — Debug+ASAN green; voxel_grid_codec_test 4/4, builtin_object_test 6/6
• pre-commit run clean (clang-format pinned v22.1.0)

Follow-ups (separate work, not in this PR)

• foxglove.VoxelGrid → sdk::VoxelGrid parser in pj-official-plugins (the zero-copy producer)
• pj_scene3D GPU-expansion renderer (dense buffer → compute-shader compaction → instanced cubes)

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