fix: Twist2.ad raised ValueError for every twist; add Twist2.Ad and SE2.Ad

[petercorke]

Problem

Twist2.ad raises ValueError: argument must be a 1- or 3-vector for every twist, and always has. It is a copy of Twist3.ad that was never adapted to 2D: it calls skew() on the scalar w and builds 6x6 blocks, where the 2D result is 3x3. Nothing in the repo calls it and no test covered it.

It also differs from its 3D sibling in other ways:

• It is a @property, while Twist3.ad is a method and both docstrings say S.ad().
• Its docstring example calls S.unit(), and its see-also points at SE3.Ad.
• Twist2 has no Ad(), and neither does SE2. tr2adjoint2 exists, but only as a base function, so the ad/Ad pair that Twist3 and SE3 have was half missing in 2D.

Changes

• Twist2.ad() is now a method returning the 3x3 matrix of the Lie bracket, [[0, -ω, v_y], [ω, 0, -v_x], [0, 0, 0]]. For a sequence it returns a list, as skewa() does.
• Twist2.Ad() is new and mirrors Twist3.Ad(): the adjoint of the equivalent SE(2) pose.
• SE2.Ad() is new and mirrors SE3.Ad().
• tr2adjoint2 docstring: its text named tr2jac2 and said 6x6. Now it names tr2adjoint2 and gives 3x3 (1x1 for SO(2)).

Verification

The new tests/test_twist2_adjoint.py checks against independent oracles, not against the implementation:

• ad(S) @ T equals the commutator of the augmented skew matrices, vee([S, T]).
• Antisymmetry: [S, S] = 0 and ad(S) T = -ad(T) S.
• expm(ad(S)) equals Ad(exp(S)).
• SE2.Ad() @ S equals vee(T [S] T^-1).
• Ad(T1 T2) = Ad(T1) Ad(T2), and Ad of the identity is I.
• Sequence handling.

All six fail on master and pass here. Full suite: 386 passed, 3 skipped. Black is clean. The branch merges cleanly with #225, #248, #249, #258 and the integration hub #246.

Not changed

• tr2adjoint and tr2adjoint2 both have a .. math: directive with a single colon (so it renders as a comment) and unbalanced braces in the formula. I left those alone because the 3D function shares them and fixing the math needs a check of the Sphinx macros.
• Twist3.ad() silently uses the first element of a sequence. Twist2.ad() returns a list, so the two now differ for sequences. I'd harmonize them separately.

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