Land T1.18: L2 in CI

contract-python per runtime cell (ref-pandas3, dbr17-pandas2, floor-
pandas2.2) and rust-test once, judged by the checkpoint harness; the
gate fails closed when it cannot decide to run; NUL-separated git paths;
contract/tests on effective expectations; H1 cell rule in compare.py
(non-reference cells gate python-core against python-old, report the
reference). floor-pandas2.2 was report-only for this candidate only
(warnings-only, anchored, bound to the absence of this landing); HOST-3
proposed. ratchet.py and gate.sh take bug fixes only from here. CI
11m35s to about 9m. E65. 3 review rounds, 2 fix rounds.

Candidate run 38196008570 green at
7571d3e; this commit changes no file,
so the phase branch moves at once and its own runs are recorded
afterwards (E43 (b)).

Reviewed-by: independent reviewer (fresh agent) and Codex, T1.18
Protected-paths: .github/workflows/ci.yml; contract/tests/test_gate_controls.py; contract/tests/test_runners.py; docs/design/rust-core/register.yaml; scripts/rust_core/compare.py; scripts/rust_core/gate.sh; scripts/rust_core/ratchet.py; scripts/rust_core/selftest.py; tests/test_contract_ci_gate.py; tests/test_contract_ratchet.py
Base-SHA: c52f280
Checkpoint-SHA: c52f280
Signed-off-by: Adi Wadaskar <awadaskar@nvidia.com>

Author: adi-kumo