feat(sensors): open lidar model factory and make LidarModel generic in its parameters

[janickm]

Applies the treatment the camera model hierarchy received in #175 / #177 to the lidar hierarchy. Same shape, so the review should feel familiar.

The closed dispatch table, twice

LidarModel.maybe_from_parameters was a static method on the abstract base holding a closed if/elif table over every concrete subclass — and StructuredLidarModel shadowed it with a second copy that had to be kept in sync by hand. Since static methods do not dispatch, which of the two tables ran depended only on the class name the caller happened to spell, and neither could construct a lidar model defined outside NCore.

Both are replaced by a module-level lidar_model_from_parameters() dispatching on the parameter type via functools.singledispatch, with maybe_lidar_model_from_parameters() for the optional case and register_lidar_model for out-of-tree models. Both static methods are kept as deprecated forwarders.

Generic model

LidarModel declared no get_parameters — only the concrete leaf did, so a LidarModel-typed value was a dead end. It is now generic in its parameter type and declares it. The parameter is covariant: it appears only in a return position, and invariance would leave concrete instantiations with no common LidarModel[...] supertype. Plain unparameterized LidarModel annotations and isinstance checks are unaffected.

Serialization on the abstract base

BaseLidarModelParameters was not even an ABC (@dataclass class BaseLidarModelParameters: pass); the JSON mixin and type() sat only on the concrete leaf, so code holding the abstract type could not serialize. It now carries both, type() non-abstract so pre-existing out-of-tree subclasses stay instantiable, and the leaf drops its own mixin.

One thing worth a look

The deprecated StructuredLidarModel.maybe_from_parameters keeps the base's parameter type rather than narrowing it to the structured parameters. Narrowing it is a contravariance violation that ty rejects once the base is widened — it was invisible before only because both copies happened to name the same concrete union. The structured-ness of the result is enforced at runtime instead.

Verified: 33/33 tests on 3.8 and 3.11, ty aspect clean, //:format.check clean.