GFQL T3.b: consolidate nullable helper call sites (#1309)

[lmeyerov]

Summary

• resolve GFQL native types T3.b: migrate inline nullable patterns onto ir.metadata helpers #1309 by migrating the two deferred inline nullable patterns onto graphistry.compute.gfql.ir.metadata helpers
• binder UNION merge now derives nullable via bound_variable_is_nullable(...)
• lowering bound nullable alias derivation now uses bound_variable_is_nullable(...)
• convert the IC1 differential scaffold trust-check to the same helper contract
• add focused binder coverage for nullable-bit preservation across UNION branch merge
• add changelog entry linking this follow-through to the T3 #1300: type/null metadata propagation contract for BoundIR/LogicalPlan seams #1304 audit context

Validation

• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_binder.py -k union_merges_branch_bindings or union_merges_nullable_bit
• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_m1_differential_scaffold.py -k trust_ic1_null_extended_from_semantics
• uv run ruff check graphistry/compute/gfql/frontends/cypher/binder.py graphistry/compute/gfql/cypher/lowering.py graphistry/tests/compute/gfql/cypher/test_binder.py graphistry/tests/compute/gfql/cypher/test_m1_differential_scaffold.py

Issue

Closes #1309