Add forward_vector_from_quaternion, direction_to_quaternion, and angle_between_directions to quaternion_utils

[Copilot]

Three new direction/quaternion utility functions in spinstep/utils/quaternion_utils.py.

• forward_vector_from_quaternion(q) — Extract forward (look) direction via R.from_quat(q).apply([0, 0, -1])
• direction_to_quaternion(direction) — Convert a 3D direction to an orientation quaternion via R.align_vectors(), with zero-vector guard
• angle_between_directions(d1, d2) — Angular distance (radians) between two direction vectors via arccos(dot) with normalization and clamping

All three are exported from utils/__init__.py and added to __all__ in both modules. 12 new tests cover identity, roundtrip, edge cases, and mathematical correctness.

from spinstep.utils import (
    forward_vector_from_quaternion,
    direction_to_quaternion,
    angle_between_directions,
)

fwd = forward_vector_from_quaternion([0, 0, 0, 1])  # [0, 0, -1]
q = direction_to_quaternion([1, 0, 0])               # quaternion aligning -Z to +X
angle = angle_between_directions([1, 0, 0], [0, 1, 0])  # π/2

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.