fix(dynamics): support arbitrary joint counts in paycap

[huxiaoyi-ovo]

Summary

Fixes #728.

The paycap() method previously allocated a fixed six-column capacity array, causing broadcasting errors for robots with other joint counts.

This PR:

• Supports arbitrary joint counts by returning per-joint capacities with shape (n,), or (m, n) for trajectories.
• Corrects trajectory length handling and normalizes each wrench independently.
• Rejects zero wrench directions.
• Handles joints with zero wrench sensitivity without division by zero.
• Updates the documentation to describe the actual return values and limiting joint.

The public method signature remains unchanged.

Tests

Added 8 parameterized regression cases covering:

• 3-, 6-, and 7-DOF robots.
• Positive and negative wrench directions.
• Multi-configuration trajectories.
• Invalid input shapes and zero wrench directions.
• Joints with zero wrench sensitivity.

Validation

Validated commit 90bc11e infork PR #1(huxiaoyi-ovo#1).

GitHub Actions completed successfully:

• Python 3.10–3.14 on Ubuntu, Windows, and macOS.
• Complete core test suite.
• Coverage and documentation builds.
• Aggregate test checks.

Result: 19 successful jobs, 1 conditional skip, 0 failures.

The patch changes only Dynamics.py and its regression test file.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 11 complexity · 0 duplication

Metric	Results
Complexity	11
Duplication	0

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (788eaf3) to head (a30a297).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/robot/Dynamics.py	0.00%	16 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #731   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        143     143           
  Lines      14375   14367    -8     
=====================================
+ Misses     14375   14367    -8     

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[petercorke]

Thank you for your interest in RTB, and for fixing #728. This is merged. While reading the history I found that the original MATLAB paycap (by Bryan Moutrie) returns the minimum over joints as a scalar capacity, and the Python port dropped that min, which is where the shape problem came from. I'll follow up with that change and a hand-checkable test, and I'll credit this PR in the commit. Thanks again.