M2-PR3: LogicalPlan verifier (structural invariants)

Parent: #1139

Objective

Add M2 structural verifier coverage for LogicalPlan and wire invariant checks into planner outputs.

Deliverables

• Add graphistry/compute/gfql/ir/verifier.py with:
  • verify(plan: LogicalPlan) -> list[CompilerError]
• Implement core invariants from milestone spec:
  • dangling reference detection
  • output schema consistency checks
  • predicate scope checks (especially PatternMatch)
  • optional-arm nullability contract checks
  • op_id uniqueness checks
• Add focused positive/negative verifier tests.

Acceptance Criteria

• Verifier catches intentionally malformed fixtures with stable error classes/messages.
• Scope checks validate true variable visibility, not placeholder/non-empty-string checks.
• Verification traversal is cycle-safe for malformed plans (no unbounded recursion).
• Verifier passes on planner-emitted plans for covered query shapes.
• CI green.

Non-goals

• No cost-based optimization logic.
• No physical planning/executor routing.

References

• plans/compiler-refactor/milestone-M2.md (verify(plan) section)
• plans/compiler-refactor/research-tracks/d4-verifier.md