feat(pj_base): add PJ.PosesInFrame builtin schema (0.8.0)

[facontidavide]

Summary

New canonical builtin type PJ.PosesInFrame — an array of poses in a single reference frame (geometry_msgs/PoseArray, foxglove.PosesInFrame, AMCL particle clouds). The schema deliberately carries no styling fields: how poses are drawn (arrows vs XYZ triads, size, color) is a viewer-side decision, matching how Foxglove (PosesInFrame + panel settings) and Rerun (InstancePoses3D + instanced geometry) model the same data.

• Proto mirrors foxglove.PosesInFrame field-for-field (timestamp=1, frame_id=2, poses=3)
• Owned-values struct PJ::sdk::PosesInFrame + hand-rolled wire codec on the shared builtin_wire Pose/Timestamp helpers
• kPosesInFrame = 17 in BuiltinObjectType and the C ABI enum, pinned by the layout-sentinel test; PJ_ABI_VERSION untouched (additive)
• Version 0.7.0 → 0.8.0 (MINOR, additive); abi/baseline.abi untouched — canary DSO verified to carry zero new symbols/DWARF entries, so the abidiff gate outcome is unchanged
• Docs: builtin_type.md section + type-erasure row, proto README, CLAUDE.md counts

Consumers (separate PRs, dependency-ordered): PJ4 PosesInFrameLayer (instanced arrows/triads with live styling) and pj-official-plugins parsers for geometry_msgs/PoseArray + foxglove.PosesInFrame.

Testing

• 45/45 tests in both build configs (RelWithDebInfo and Debug+ASAN); codec suite covers schema name, empty buffer, 2-pose round-trip incl. the proto3 w=1 default-quaternion edge, empty poses, and truncated nested-message error
• pre-commit clean; independently reviewed (spec-compliance + code-quality passes, findings addressed)

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