Harden the i472 trajectory storage-contract gate: finiteness/type-check the 6 raw-logit leaf fields + AST gate-before-write ordering pin

Follow-up capture from #576 round-2 reconciler (epm:review-reconcile v2, deferred concern trajectory-gate-presence-only-validation).

The storage-contract gate added in #576 r2 (assert_trajectory_slot_records_meet_storage_contract, src/explore_persona_space/experiments/contrastive_neg_geometry_472/eval_trajectory.py) is presence-only (leaf.get(k) is None): a NaN raw-logit value (corrupted adapter/forward pass) survives float(tensor.item()) and persists in the final canonical artifact. Non-numeric strings are unreachable (single producer emits float(...item())), so this is the NaN half only — adjudicated real-but-non-blocking, low recoverable cost (adapters persist on HF).

Asks (reconciler standing recommendations 1-2, ~3 lines + 1 test):

1. Extend the gate to require each of the 6 TRAJECTORY_LOGIT_LEAF_KEYS be a finite, non-bool float (mirror _is_finite_number in eval/marker_logprob.py). Alternative one-line backstop noted: allow_nan=False on the final json.dumps.
2. Extend the AST pin in tests/test_marker_slot_contract.py to assert the gate call precedes out_path.write_text inside run_trajectory_eval (currently pins call count only).
3. Optional minors carried from review: assert checkpoints against degenerate empty input.

Constraint: validate_marker_slot_record semantics stay untouched (live consumers #542/#597).