fix(cypher): restore free-form reentry plan contract post-#1260 - #1314

Summary

• Worker D Cypher/GFQL: introduce a first-class row-carrier IR for multi-stage vectorized row semantics #989 follow-through cleanup slice (dead-path retirement tied to post-Meta: GFQL monolith shrinkdown (lowering.py + CompiledCypher cleanup) #1260 reentry extraction)
• Fix bounded reentry plan construction so free-form intermediate MATCH compiles as ReentryPlan(free_form=True, scalar_only=False) instead of degrading to scalar-only tagging
• Keep reentry attachment explicit on the terminal reentry consumer and remove incidental free-form/scalar-only branch coupling
• Add compile-shape regression lock for free-form plan contract
• Update changelog and scalar-only runtime helper comment to match the new contract

Validation

• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "compile_cypher_records_freeform_reentry_plan_contract or freeform_intermediate_reentry or scalar_only_prefix_with_match_reentry"
• python -m pytest -q graphistry/tests/compute/test_gfql.py -k "reentry"
• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_cycle_policy.py
• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "reentry"
• uv run ruff check graphistry/compute/gfql/cypher/reentry/runtime.py graphistry/compute/gfql_unified.py graphistry/tests/compute/gfql/cypher/test_lowering.py

Issue

• Cypher/GFQL: introduce a first-class row-carrier IR for multi-stage vectorized row semantics #989