fix: don't pass a positional argument to pgraph.UGraph

[petercorke]

Summary

• `BundleAdjust` built its graph with `pgraph.UGraph(6)`. pgraph 1.0 makes constructor arguments keyword-only, so this raises `TypeError` there.
• pgraph 0.6.5 accepted the `6` and silently ignored it, so the "6D coordinates" the comment describes were never enforced. They couldn't be: view vertices are 6D and landmark vertices are 3D (`dim=6` would reject every landmark).
• Use `pgraph.UGraph()` and fix the comment. Behaviour is identical on 0.6.5 and 1.0, so this can merge before or after pgraph 1.0 is released and needs no version pin.

Test plan

• `tests/test_bundle_adjust.py` and `tests/test_bundleadjust.py` pass (4 tests) against the pgraph 1.0 development branch with keyword-only arguments

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

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