fix(ik): unify ik_XX/ikine_XX return types and fix IK docs

[petercorke]

Summary

• ik_LM/ik_NR/ik_GN (fast, C++-backed) returned a bare 5-tuple while ikine_LM/ikine_NR/ikine_GN/ikine_QP (pure-Python) already returned IKSolution -- same family of methods, two different return shapes. Wrapped the C++ tuple in IKSolution (in ETS.py and RobotKinematics.py's forwarders), and updated return-type annotations and :returns:/:rtype: docs to match.
• Added IKSolution.__getitem__ (matching __iter__'s order) and a __repr__ matching the existing __str__ -- previously sol[0] raised TypeError, and the default dataclass repr was inconsistent with __str__.
• Added a loud .. warning:: to ik_LM/ik_NR/ik_GN's docstrings: they require the compiled C++ extension and raise RuntimeError without it (pure-Python builds, Pyodide/JupyterLite) -- use ikine_LM/ikine_NR/ikine_GN instead there.
• Made every ik_XX/ikine_XX pair bidirectionally cross-reference each other (previously ik_XX only referenced its C++ siblings, and vice versa).
• Fixed "Levemberg-Marquadt"/"Marquadt" -> "Levenberg-Marquardt" (widespread misspelling across IK.py, RobotKinematics.py, DHRobot.py, and two docs files) plus a few other typos ("deined" -> "defined", "progamming" -> "programming").
• Found and fixed several copy-paste bugs while doing this review: RobotKinematics.ik_GN's own "See Also" listed itself instead of ik_LM/ik_NR; ikine_GN's and ikine_QP's "See Also" referenced the wrong solver class entirely (IK_NR instead of IK_GN/IK_QP); two runblock examples said "ikine_GN"/"ikine_LM" in prose while actually calling ik_NR/ik_LM/ik_GN.
• Found and removed 5 completely broken, dead methods: DHRobot.ik_lm_chan/ik_lm_wampler/ik_lm_sugihara/ik_nr/ik_gn all forwarded to self.ets().<same-name>(...), but ETS has never had methods by these names -- every one of them raises AttributeError unconditionally, confirmed by direct testing. Their docstrings even had literal :seealso: TODO placeholders. No test exercised any of them.
• Fixed examples/ik_exp.py, the only caller of those dead ETS-level method names (ets.ik_nr/ets.ik_gn/ets.ik_lm_chan/etc.), to use the real ik_NR/ik_GN/ik_LM(method=...) API -- also fixed a 5-tuple unpacking that would have silently broken against the new 6-field IKSolution, and dropped several unused imports (fknm, swift, spatialgeometry, sys).

Item 3 of a 4-item IK-solver cleanup plan (see claude-notes/ik-solver-cpp-python-divergence.md). Item 1 (YuMi gripper parent-swap) is PR #649, Item 2 (broken example references) is PR #650.

Test plan

• New regression tests: IKSolution.__getitem__/__repr__, and ik_LM/ik_NR/ik_GN return real IKSolution instances (isinstance checks)
• Full test suite green (731 passed, 18 skipped) in a clean isolated venv, confirmed both before and after the dead-method removal
• Confirmed (by actually reverting each fix) that the 5 dead DHRobot methods really do raise AttributeError unconditionally, and that ik_exp.py runs end-to-end after the rewrite
• Sphinx docs build (-W --keep-going) clean of any new warnings -- only the same 2 pre-existing, unrelated roboticstoolbox.tools.trchain import warnings remain

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (6691113) to head (6d85262).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/ets/ETS.py	0.00%	7 Missing ⚠️
src/roboticstoolbox/robot/IK.py	0.00%	4 Missing ⚠️
src/roboticstoolbox/robot/RobotKinematics.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #652   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
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  Files        143     143           
  Lines      14026   14027    +1     
=====================================
- Misses     14026   14027    +1     

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