Remove residual Cypher chain fallback - #1508

Closes #1486.

Summary

• reran the post-Plan path-alias carry matches natively #1503 residual fallback inventory on latest master and confirmed no approved-chain defer-code hits remain
• removed the final non-union logical_plan is None approved chain-execution branch and its runtime allowlist/helper
• added stable logical-plan defer-code symbols and fail-fast coverage proving multiple_match_stages, optional_match_reentry, and unknown defer codes no longer silently chain-execute
• changed missing-logical-plan runtime errors to expose structured logical_plan_defer_code / logical_plan_defer_reason context keys instead of a nested value payload
• updated CHANGELOG.md

Inventory

Base after rebase: origin/master @ f1831304

candidate_literals=2386
unique_candidate_literals=1847
compiled=1009
compile_errors=838
logical_plan_none_without_code=11
defer_codes=none

The remaining logical_plan_none_without_code examples are UNION programs; _execute_compiled_query() dispatches CompiledCypherUnionQuery before non-union execution, so they do not reach _execute_compiled_query_non_union().

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/test_runtime_physical_cutover.py -k 'not cudf' graphistry/tests/compute/gfql/test_logical_planner.py graphistry/tests/compute/gfql/cypher/test_lowering.py::test_string_cypher_executes_seeded_multihop_then_with_optional_match_reentry_shape
57 passed, 1 deselected
• python3 -m pytest -q graphistry/tests/compute/gfql/test_runtime_physical_cutover.py::test_non_union_execution_requires_logical_plan graphistry/tests/compute/gfql/test_runtime_physical_cutover.py::test_deferred_logical_plan_reason_rejected_before_chain_execution graphistry/tests/compute/gfql/test_runtime_physical_cutover.py::test_deferred_logical_plan_code_rejected_before_chain_execution
5 passed
• python3 -m pytest -q graphistry/tests/compute/gfql/test_runtime_physical_cutover.py -k 'not cudf'
30 passed, 1 deselected
• python3 -m pytest -q graphistry/tests/compute/gfql -k 'not cudf'
2167 passed, 29 skipped, 134 deselected, 15 xfailed
• python3 -m ruff check graphistry/compute/gfql/defer_codes.py graphistry/compute/gfql/logical_planner.py graphistry/compute/gfql/cypher/lowering.py graphistry/compute/gfql_unified.py graphistry/tests/compute/gfql/test_runtime_physical_cutover.py
  passed
• ./bin/typecheck.sh
Success: no issues found in 273 source files
• local full test_runtime_physical_cutover.py cuDF case fails on this host before assertions with cudaErrorNoDevice; covered on DGX
• dgx-spark RAPIDS 25.02 focused GFQL/cuDF runtime cutover file
  31 passed, 1 warning, log /tmp/issue-1486-rapids-25.02.log
• dgx-spark RAPIDS 26.02 focused GFQL/cuDF runtime cutover file
  31 passed, log /tmp/issue-1486-rapids-26.02.log

Merge

Auto-merge after DGX validation, review-skill convergence, CI green, and final master freshness check.