fix(circle_layout): entry guards for edge-only graphs, missing positions, and duplicate bounding_box keys

[SAY-5]

Summary

• Linked issue(s): Closes layout: circle_layout entry guards — TypeError on edge-only graphs (len() before materialize_nodes), bare KeyError('x') for the no-arg form, IntCastingNaNError on duplicate bounding_box keys #1968
• Scope is explicit (non-goals included when relevant)

Fixes the three entry-guard crashes from #1968: nodes are now materialized before being counted so an edge-only graph lays out instead of hitting TypeError: object of type 'NoneType' has no len(); the no-argument form on a graph without x/y positions raises a ValueError naming the documented precondition instead of a bare KeyError: 'x'; and a partition-keyed bounding_box frame with duplicate partition_key values raises a ValueError naming the duplicates instead of a pandas IntCastingNaNError from the row-multiplying merge. Non-goals: defects 4 and 5 from the issue (the misleading null-partition-key message and the dead engine comparison) are left untouched.

Validation

• Local lint/type/tests run for touched scope (ruff clean, mypy clean for circle.py, graphistry/tests/layout green including the new test_circle_entry_guards.py)
• CI is green

Cypher Frontend CI Evidence (when PR touches cypher frontend / IR scope)

Not applicable — layout-only change.