Solution charts: remaining work after #579

[siddhss5]

#579 lands charts of the self-motion manifold: SelfMotionManifold / Chart, q(t), tangent by implicit differentiation, frame(t, metric), in_limits subdomains, arc-length length / sample, sheets / gap / escape / locate, branch label and tracking with monodromy, solve_path, a native mirror, and 0-D charts for ikgeo.three_parallel.

This issue is what remains.

API gaps

• pullback_metric(t, metric): expose g_phi = Dphi^T G Dphi. length and sample already compute it internally, and it is what a null-space-optimization caller needs.
• Rename the accessor: Manipulator.charts(T) returns a manifold, so callers write arm.charts(T).charts(). Prefer arm.self_motion(T).

Coverage gaps

Charts today exist only for seven_r.spherical_shoulder, seven_r.srs and ikgeo.three_parallel. Everything else raises NotImplementedError.

• Jointlock: needs implicit differentiation of the unlocked 6R system. Blocked on a labelling question, since jointlock branches carry no stable label across lock samples.
• Polished / approximate-SRS: differentiate implicitly at the corrected solution rather than through every LM iteration.
• ikgeo.general_6r, Husty-Pfurner, spherical_two_parallel: no chart at all. Relevant to M7, which targets general_6r (see Define independent IK soundness, recovery, and branch-completeness contracts #573).

Contracts to settle

• label semantics: universal stable identity, or explicitly family-scoped? The tracking design depends on it, and the families above cannot all provide the same guarantee. Decide before more consumers rely on it.
• Metric catalogue: which ship, identity, constant weighted SPD, configuration-dependent inertia.
• What "differentiable" means: analytic Jacobian only, or JAX / PyTorch compatibility too. Root sorting, dedup and branch creation are not globally smooth, so autodiff through the solver is a separate question from exposing Dphi.
• Serialization and the native boundary: Add self-motion charts #579 answers this de facto with a Python/C++ wrapper object. Ratify, or decide whether generated native artifacts should expose the same ABI directly.

Cross-cutting

• Terminology: General 6R: multi-chart reconstruction and end-to-end representation selection #572 uses "multi-chart" for an algebraic coordinate chart, unrelated to Chart here. Rename one before both exist in the codebase (General 6R: multi-chart reconstruction and end-to-end representation selection #572 is cheaper).
• Centralize public SolveOptions validation and semantics #575 owns promoting respect_limits: bool | Literal["wrap"] to a typed enum, with bool kept as a deprecated alias.