refactor(ik): joint limit helpers as module-level functions

[petercorke]

Summary

• Move the joint limit normalisation and check out of IKSolver into module-level normalise_q(q, qlim, revolute) and within_limits(q, qlim) in roboticstoolbox/robot/IK.py. They take plain arrays instead of an ETS.
• IKSolver._normalise_q and _check_jl become thin wrappers that map the jindex-indexed coordinates onto the ETS joints and call the helpers.
• Purpose: let other IK implementations, e.g. the analytic IK being developed for DHRobot (which has qlim and joint types but no ETS), share the numerical solvers' coordinate convention (keep in-limit values, otherwise principal angle shifted by whole turns toward the limits) instead of duplicating it.

Behaviour

Unchanged, including that a NaN coordinate is not reported as outside its limits (same q < lo or q > hi semantics as before). No public API change: the new functions are not exported from the package.

Test plan

• tests/test_IK.py and tests/test_ik_joint_limits.py pass unchanged (121 passed, 12 skipped)
• New direct tests of the helpers: in-limit values preserved, shift into limits, principal angle preferred, input not modified, prismatic untouched, no-equivalent-angle rejected, exact limit endpoint after whole turns, inclusive limit check
• Differential test, old methods from main vs the new wrappers, 7200 random cases over 6 jindex layouts (default, identity, reversed, permuted, sparse) and 3 limit sets: identical results
• CI

🤖 Generated with Claude Code

[codacy-production]

Not up to standards ⛔

🔴 Issues 7 high

Alerts:
⚠ 7 issues (≤ 0 issues of at least minor severity)

Results:
7 new issues

Category	Results
Security	7 high

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🟢 Metrics 8 complexity · 0 duplication

Metric	Results
Complexity	8
Duplication	0

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (acc6842) to head (a0478ef).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/robot/IK.py	0.00%	32 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #713   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        143     143           
  Lines      14269   14275    +6     
=====================================
- Misses     14269   14275    +6     

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