[C++ backend] MoveIt wrapper + plugin packaging (#554 D)

[siddhss5]

Sub-issue of the native C++ backend umbrella #482.

Explicitly not a phase gate. A generated artifact and a MoveIt plugin test different things: ROS packaging, plugin loading, frame conventions, consistency limits, callbacks, redundant-joint handling, and searchPositionIK timeout semantics. q_seed + max_solutions alone do not implement the full MoveIt contract.

Scope:

• A generic kinematics::KinematicsBase subclass wrapping native solve/solveInto — getPositionIK / searchPositionIK (seed + timeout) / getPositionFK.
• Plugin-description XML, package.xml, and a per-arm plugin generator.
• MoveIt-side conformance (frames, redundant joints, timeout behavior) separate from the artifact conformance ([C++ backend] Conformance harness (5 levels + redundant-7R split) #487).

Note: OMPL has no single universal IK plugin interface; it consumes IK via application-specific state samplers / constraint machinery — a separate integration from bare C++ usability, not covered here.

[siddhss5]

Deferred / de-emphasized. Gated on the bulletproof-C++ milestone (Phases 0a-3 conformance green). MoveIt integration (ROS packaging, plugin loading, frames, consistency limits, searchPositionIK timeouts) is a separate contract from artifact correctness and should not run in parallel with hardening the solver families.

[siddhss5]

Bare-C++ consumability shipped (#527). Header-only ssik::ssik_cpp INTERFACE target with install(EXPORT) + configure_package_config_file, so an external C++/MoveIt project does find_package(ssik_cpp) + target_link_libraries(... ssik::ssik_cpp) and calls ssik::<arm>::solve(T) with zero Python. Validated end-to-end by a standalone consumer smoke in CI (install to a temp prefix -> find_package -> build -> run).

The MoveIt-specific work (KinematicsBase plugin, searchPositionIK seed + timeout semantics) stays DEFERRED until all arms are native and the parallel batch API lands.

[siddhss5]

Deferral lifted — the "bulletproof C++" precondition is met.

All 72 arms are native and parity-gated (#556/#558), the native path is crash-hardened (exception-safe parallel_for + -DNDEBUG, #560), and native=True is now the default (#560, v5.0.0). Every arm ships a zero-runtime-Python cpp/gen/<arm>.hpp with the full solve() contract.

Ready to build on: the bare-C++ consumer path + docs are #561; this issue is the MoveIt KinematicsBase plugin layer on top of that. Scope below is unchanged.

Part of the #554 "effortless C++" pillar (D), alongside #561 (consumer docs) and #492 (RoboPlan adapter).

[siddhss5]

Distribution model: ssik-hosted plugin generator, NOT an upstream PR into MoveIt.

MoveIt core does not host per-solver IK plugins — it ships the kinematics::KinematicsBase interface + a generic template, and each solver (e.g. IKFast) provides a generator that emits a per-robot plugin package the user builds into their own moveit_config. ssik follows the same pattern:

• Host an ssik_moveit companion (in the ssik org) with the KinematicsBase wrapper over <arm>::solve/fk + a per-arm plugin generator (emits the subclass + plugin_description.xml + package.xml).
• Users generate a plugin for their arm and build it; nothing is PR'd into moveit.

A generic "ssik template" upstream into moveit_kinematics (like the ikfast one) is a possible long-term follow-up, but it carries version coupling and is not the path to shipping. Builds on the bare-C++ consumer path/docs (#561).