fix: check numerical IK convergence before solver step

[tahazarif10]

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Summary

Fix the shared numerical IK solve loop so an initial configuration that already satisfies the target pose is accepted before attempting a solver update.

Currently, IKSolver._solve() calls step() before checking whether the current configuration has already converged. This can cause an unnecessary linear algebra failure even when q0 is already an exact solution — notably with a redundant Panda robot using IK_LM(method="chan").

This change:

• evaluates the residual for the initial configuration before any solver update;
• checks convergence before attempting the next step;
• makes NR, LM, GN, and QP step() implementations return the residual associated with the updated configuration;
• preserves existing joint-limit and search behavior;
• adds a regression test covering exact-q0 convergence for LM, NR, and GN with zero solver iterations.

Related issue

Fixes #664

Checklist

• PR title follows Conventional Commits
• Tests pass locally (pytest)
• Added/updated tests for this change, if applicable
• New/changed code is type-hinted where applicable
• Docstrings remain consistent with the updated solver contract
• PR is small/focused on a single issue
• No project-specific test files, data files, or notebooks are included

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (b53f255) to head (8b0eb83).

Files with missing lines	Patch %	Lines
src/roboticstoolbox/robot/IK.py	0.00%	15 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #667   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        143     143           
  Lines      14050   14055    +5     
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- Misses     14050   14055    +5     

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[petercorke]

Thanks @tahazarif10 for the fix, and @niamorg for originally flagging this. Glad to see people are using the IK functions. While looking at this fix I found yet another thing to fix, see #668.