feat(ets): add trchain and trchain2 transform evaluators

[Tobias-Fischer]

Summary

• add Python trchain and trchain2 evaluators for compact SE(3) and SE(2) transform chains
• support joint variables, degree/radian units, named and symbolic expressions, and reusable parsed tokens
• expose the functions at the toolbox top level and document them alongside ETS with runnable examples

Motivation

The Robot Academy videos currently lack a Python implementation for trchain/trchain2. EGB339 students learn this transform-chain notation, so it would be useful to have the same functionality in RTB-Python.

Validation

• pytest tests/test_tools.py -q — 7 passed
• Ruff lint and format checks passed
• Python byte-compilation passed
• Sphinx HTML documentation rendered the new section, examples, and API links

[codecov]

Codecov Report

❌ Patch coverage is 0% with 96 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (5977ac4) to head (4ecff40).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/tools/trchain.py	0.00%	95 Missing ⚠️
src/roboticstoolbox/tools/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #615   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        142     143    +1     
  Lines      13898   13994   +96     
=====================================
- Misses     13898   13994   +96     

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