ETS/ETS2: one joint-index resolver; the all-or-none, unique guarantee breaks under concatenation and insert #803

Summary

The rule "an ETS has either all or none of its joint indices set, and they are unique and contiguous" is implemented four times and enforced in only one place, the constructor. Incremental construction and later mutation can break it.

Found while reviewing #792, which fixed one symptom (np.all([...jindex]) treats index 0 as missing, so a permuted or sparse mapping was silently overwritten with 0..n-1).

Where the logic lives today

The guarantee does not hold under incremental construction

On main:

a, b = ET.Rz(), ET.Rx()
e = ETS([a]) * ETS([b])       # each piece was auto-assigned jindex 0
[j.jindex for j in e.joints()]   # -> [0, 0], n = 2: all set, but not unique

• Concatenating two auto-indexed ETS gives duplicate indices with no error, because the constructor only checks "all or none", not unique and contiguous.
• ETS.insert appends to _data with no re-check at all and does not reset _auto_jindex.
• ET.jindex has a public setter, so indices can be changed after construction, outside the owning ETS (Link.jindex, BaseRobot, Gripper, DHRobot and ETS2.jacob all do).

Proposal

1. One helper (a function, or a method on the common base BaseETS) that takes the joints and does the whole job: all-or-none check, uniqueness and contiguity, auto-fill, and returns whether the indices were auto-assigned. ETS and ETS2 constructors call it.
2. Re-run it from every mutator (insert, append, __setitem__, __imul__) or resolve it lazily on first use, since jindices is already a cached property.
3. Make ET.jindex read-only once the ET belongs to an ETS (the ET is already deep-copied into the ETS, so ownership is clear). Mutation then goes through one sanctioned ETS method that keeps the _auto_jindex flag and the cached jindices consistent. Link.jindex, BaseRobot, Gripper and DHRobot use that method.
4. BaseRobot and Gripper share a small classifier (none / all / mixed) instead of each re-writing all([... is None ...]), and their DFS assignment becomes one base-class method.
5. The unreachable renumbering block in ETS2.jacob is deleted.

Regression tests: concatenation of two auto-indexed ETS, insert of a joint, permuted explicit indices surviving jacob, and a mixture still raising.

Fits the ets group of the typing pass (problems 61 to 65).

Related

• fix(ets2): preserve joint indices and flipped Jacobian signs #792 (the symptom fix, independent of this)