fix: honour flipped standard-DH joints in DHLink.A

[tahazarif10]

Fixes #563

Summary

Use the variable joint ET (self.v) when applying flip in DHLink.A() instead of assuming the last ET in the sequence is the joint ET.

For standard DH links, constant transforms such as d, a, or alpha can follow the joint ET, so self.ets[-1].isflip may inspect the wrong ET.

Tests

Added regression coverage for both revolute and prismatic standard-DH links with flip=True.

• python -m pytest tests/test_DHRobot.py -k test_A_flip_standard_dh -q
  • 1 passed
• python -m pytest tests/test_DHRobot.py -q
  • 71 passed, 1 skipped