feat(pj_base): add GridMap canonical builtin object (SDK 0.26.0)

[facontidavide]

Summary

Adds sdk::GridMap, a 2D grid whose cells carry named channels: the layered, generic-valued sibling of OccupancyGrid for elevation maps and multi-layer costmaps (grid_map_msgs/GridMap, foxglove.Grid). First of three PRs for rendering grid maps as a lit, colormapped heightfield in PlotJuggler 4's 3D view; the PJ4 renderer and the parser mappings follow once this is released.

Design: row-major fixed-size cell records described by the shared PointField channel model, mirroring foxglove.Grid plus an explicit row_count, so Foxglove producers hand data over zero-copy and only ROS grid_map needs a one-time transcode. A NaN in a float channel means "no data". Which channel is height, which drives color, and the colormap are viewer-side; the type carries no styling. Conventional channel names documented: elevation, and red/green/blue/alpha for RGBA.

Changes

• pj_base/builtin/grid_map.hpp and GridMap.proto (data = field 10)
• grid_map_codec.{hpp,cpp}: decode rejects a layout the cell math could not index safely (zero stride with cells declared, row shorter than its columns, data shorter than row_count * row_stride, a field reaching past cell_stride)
• BuiltinObjectType::kGridMap = 20, PJ_BUILTIN_OBJECT_TYPE_GRID_MAP = 20, splice-table entry, type-erased dispatcher cases
• sdk::Vector2 in the geometry vocabulary with wire helpers (PJ.Vector2 already existed on the wire)
• Docs: builtin_type.md section plus classification and conversion rows, proto README, builtin counts in both CLAUDE.md files
• VERSION 0.26.0 and CHANGELOG entry. MINOR per the release policy: additions only, no existing struct, slot, or wire format changes.

Test plan

• Tests written first: 7 new GridMapCodecTest cases (round trip with NaN preserved, ownership, empty grid, four rejections) plus the enum, ABI-sentinel, metadata and splice-table tests extended; they failed on kGridMap undeclared before the implementation
• Release build + ctest: 84/84
• ./build.sh --debug && ./test.sh (ASAN): 84/84
• pre-commit clean
• CI abi_check (abigail not installed locally; additions only)

After merge

Release 0.26.0. Not included, deliberately: a GridMapBuilder in the WASM parser-module kit and a validity-mask (basic_fields) extension; both are additive if wanted later.

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