fix: broken docstring examples in Twist2/Twist3/DualQuaternion (and their shared root cause)

[petercorke]

Summary

Found via the sphinx-pyrunblock rollout (#221): several .. runblock:: docstring examples were silently baking broken tracebacks into the published docs instead of demonstrating real output.

Twist2.unit() / Twist3.unit() -- branches were swapped, self.w had a S.w typo, and the "prismatic" branch passed a wrong-shape zero argument for Twist2 (3-element list where a scalar is required).

Twist2.pole -- the docstring's own example called S.pole(), but pole is a @property, not a method.

DualQuaternion: a shared root cause across three methods -- fixing SE3()'s docstring (called d.T instead of the actual d.SE3() method) surfaced that UnitQuaternion.conj() silently re-canonicalized its result's sign, which is correct for constructing a UnitQuaternion (q and -q are the same rotation) but wrong for conjugation, an algebraic operation that must satisfy q*conj(q) == 1. Fixed Quaternion.conj() itself rather than routing around it at each call site. This one fix resolved three separate, previously-untested bugs:

• SE3(): silently returned a transform with negated translation for any rotation with negative quaternion scalar part.
• norm(): crashed outright (math domain error) for the same reason -- also added a small floating-point clamp for the mathematically-non-negative-but-can-round-negative norm-squared terms.
• Vector transformation (dq * v): unrelated to the sign bug -- affected every case regardless of sign. The textbook q*P*conj(q) sandwich product's translation terms cancel to exactly zero under this class's own dual-part embedding convention (dual = 0.5*Pure(t)*real). Reused the already-correct SE3() extraction instead of hand-deriving a second, convention-specific sandwich formula.

Test coverage

All of the above had zero prior test coverage -- existing DualQuaternion tests used SE3.Rx(pi/4) (no translation, positive quaternion scalar part), which can't exercise any of these bugs. Added regression tests for conj()'s sign behaviour, both branches of Twist2/Twist3.unit(), Twist2/Twist3.pole, and DualQuaternion's SE3()/norm()/vector-transform.

Test plan

• Full suite: 334 passed (up from 326), 0 regressions
• Real sphinx-build verification (isolated venv, before/after this branch): RUNBLOCK-ERROR count 6 -> 0, warning count unchanged at 14 (confirmed stable across repeat builds of both branches, ruling out build-to-build nondeterminism as an explanation either way)