v0.4.0

Laya-MLX v0.4.0 selectively ports upstream Laya v0.4.1 runtime features to native MLX inference.

## Behavior change

`Router()` now sends language-undecided inputs to the multilingual checkpoint. Identified English still uses the English checkpoint. Set `Router(default="english")` to retain the previous fallback behavior.

## Changes

- Keep resident-model requests and status reads available during cold checkpoint construction. Deduplicate builds and synchronize targeted unloads per checkpoint.
- Register custom checkpoints with `register`, `unregister`, `registered`, or `models=`. Source replacement and unregistration cannot resurrect an outdated in-flight model.
- Add `predict_tournament` to narrow large choice sets without an embedding model. Its probabilities and usage describe the final pass; metadata records finalists and elimination rounds.
- Add scalar and per-option-count `min_confidence` gates to Agent and Router. Gate results are reported without replacing the original answer or automatically invoking a fallback.
- Use FP32 intermediate computation for CPU parallel-option attention to avoid native FP16 kernel crashes on hosted macOS 26; GPU precision and parameter storage precision are unchanged.
- Support checkpoints trained with `option_layout="parallel"` throughout MLX inference, compilation, prefix caching, and conversion. Existing checkpoints remain sequential; do not enable parallel layout by editing an old checkpoint's configuration.
- Fix language detection for capitalized acronyms, address fragments, emphasis capitals, and mixed scripts; fix French device-footer removal.
- Release MLX free-buffer caches on unload and eviction while preserving caller-owned and in-flight model references.
- Update English/Chinese documentation, attribution, and CI with separate pinned sequential and parallel-layout upstream references.

## Validation

- 231 tests passed on CPU and 231 on GPU before release.
- Three published checkpoints in FP32 and FP16: 378/378 argmax agreements with the pinned sequential upstream reference.
- 100 deterministic finite calls per configuration, 600 total, with zero measured active-memory growth.
- Parallel-layout tests cover all 24 permutations of four options in FP32/FP16, numerical comparison with upstream v0.4.1, compilation, caching, and export/reload.
- Ruff, wheel/sdist strict metadata checks, and installed-wheel smoke tests passed.

[Raw checkpoint validation](https://github.com/mizorewww/laya-mlx/blob/v0.4.0/benchmarks/results/upstream-v041-validation.json) · [API and migration details](https://github.com/mizorewww/laya-mlx/blob/v0.4.0/README.md#upstream-v041-runtime-port)

Install or upgrade:

```bash
python -m pip install --upgrade laya-mlx==0.4.0
```