Remove Cypher reentry compiletime lowering shim - #1474

Fixes #1471.

Summary

• Removed the broad globals().update(vars(lowering)) symbol-table shim from graphistry.compute.gfql.cypher.reentry.compiletime.
• Replaced it with explicit imports for the bounded-reentry compile-time dependencies while leaving the separate projection_planning.py compatibility shim out of scope.
• Added split-guard coverage so reentry compiletime cannot reintroduce a broad lowering symbol-table rebind and must retain the lowering-called compiletime entrypoints.
• Updated the co-import seam comment.
• Added a CHANGELOG.md Development/Internal entry.

Validation

• python3 -m compileall -q graphistry/compute/gfql/cypher/reentry/compiletime.py
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py graphistry/tests/compute/gfql/test_ir_type_propagation.py::TestSeamWith1303LoweringSplit::test_post_1303_modules_and_t3_helpers_coimport — 4 passed
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "reentry and not cudf" — 114 passed
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py graphistry/tests/compute/gfql/cypher/test_lowering.py -k "reentry and not cudf" — 116 passed after entrypoint-guard test addition
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_binder_cross_kind_rebind.py graphistry/tests/compute/gfql/cypher/test_cycle_policy.py — 32 passed
• python3 -m ruff check graphistry/compute/gfql/cypher/reentry/compiletime.py graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py graphistry/tests/compute/gfql/test_ir_type_propagation.py
• python3 -m ruff check graphistry/tests/compute/gfql/cypher/test_lowering_s3_split_guard.py
• ./bin/typecheck.sh — no issues in 262 source files
• git diff --check
• python3 -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "not cudf" — 942 passed, 12 skipped
• dgx-spark RAPIDS 25.02 GFQL image focused cuDF reentry matrix (--maxfail=0, 8 tests) — 8 passed, log: /tmp/issue-1471-rapids-25.02.log
• dgx-spark RAPIDS 26.02 GFQL image focused cuDF reentry matrix (--maxfail=0, 8 tests) — 8 passed, log: /tmp/issue-1471-rapids-26.02.log