[snkmcb]

What changed?

Live gaze directions cannot use the existing point-only LookAt API without inventing a target distance or introducing eye-offset parallax. Add LookAtEvaluator::EvaluateDirection for a world unit vector, head orientation and explicit timestamp, using the existing owner angle/range-map/output implementation for both bone and expression rigs.

Directions ignore head translation, avatar/clip eye offsets and the point-only distance threshold. Invalid vectors resolve to no gaze with diagnostics. VRMRIG_LOOKAT_DIRECTION_API advertises the additive entry point for separately built consumers. Update the library documentation and motion policy.

Validation

• Tests or checks were run, or the reason they were not run is described below.
• Documentation was updated when needed.
• I did not add private or non-redistributable assets.

Checks run on Windows x64/MSVC 19.51 with OpenUSD 26.08 and installed motionCore 0.5.0:

• Standalone Release vrmRig configure/build/install succeeded.
• vrmRig_unit and vrmRig_boundaries: 2/2 passed, including signed direction yaw, both output types, invalid vectors, timestamp/diagnostics and independence from placement/offsets/distance threshold.
• Separately built usd-avatar-runtime adapter against the installed owner package: 5/5 contract/adapter tests passed, including its separately configured installed consumer.
• Existing dependency-free runtime direct suite: 3/3 passed.
• Relative documentation links/fragments and git diff --check passed.

Notes for reviewers

Existing value layouts and point/pose overloads remain unchanged. Consumers using the new entry point need a rebuilt owner library as well as updated headers. This does not publish a new package version or change clip MotionPose::lookAtTarget point semantics.

The companion runtime PR, animu-sphere/usd-avatar-runtime#6, connects world/joint-local point/direction observations to this owner API. Merge/rebuild/install this owner change before adopting the runtime adapter change. Integration evidence uses constructed rigs and test input; real avatar/motion/connector acceptance remains open.