Add registry-wide model contract gates - #1208

Closes part of #1156.

Summary

Turn a focused subset of Braindecode's model conventions into a registry-wide, machine-enforced first validation layer instead of another manual checklist.

The gate is intentionally test-only and reuses the existing models_mandatory_parameters registry, so every newly registered model is exercised automatically without a second opt-in list.

Enforced contracts

• eval-mode forward must not mutate the input tensor;
• after legitimate lazy first-forward materialization, eval-mode inference must not mutate persistent model state;
• reordering independent batch samples must reorder batched outputs in the same way (batch-permutation equivariance);
• changing one batch member while holding another fixed must not change the fixed sample's outputs (batch-composition invariance / cross-sample leakage guard);
• changing one batch member must not change another member's eval-mode outputs (batch-composition invariance), catching symmetric cross-sample leakage that permutation equivariance alone cannot detect;
• changing one batch member must not change another member's eval output (batch-composition invariance), which catches symmetric cross-sample leakage that permutation equivariance alone cannot detect;
• model outputs must contain tensor data that preserves the batch dimension;
• floating/complex outputs must be finite;
• get_config() must be JSON serializable;
• from_config(config) + state_dict must reconstruct the same eval-mode tensor output structure and values.

The state-purity and permutation checks share one second forward pass. A third composition probe keeps sample 0 fixed while perturbing only sample 1; this catches symmetric cross-sample mixing that permutation equivariance alone cannot detect.

Failure policy

The common gate is fail-closed for newly registered models. Legitimate task-specific exceptions should be explicit data with a reason rather than ad-hoc skips, so CI can distinguish a real model-contract violation from an intentionally unsupported contract.

Validation

The exact current head ebe5f1a75f54dd02d92185b80154ee484596aa0f passes 142 contract cases with 2 existing/intentional skips and no model-specific exception skips. This includes the new cross-sample leakage probe across the full registered-model matrix. Independent run: https://github.com/lindicaphxag-tech/lindicaphxag-tech/actions/runs/37206037736

Non-goals

This is the first shared validation layer, not a replacement for model-specific fidelity testing. Pretrained/reference parity, task-specific output contracts, and broader variable-length/channel guarantees remain separate, more expensive layers and can be added independently without making this gate heavy.