…ntions

ZUNA worked CUDA-only because flex_attention is unstable for autograd on
CPU; this collapses the whole API onto SDPA and trims the inference
port to a maintainable size.

Model (braindecode/models/zuna.py, 861 -> 400 lines):
- Drop flex_attention path, BlockMask, _create_document_mask, packed
  multi-document layout, and the use_flex / attention_impl plumbing.
  SDPA per-batch is mathematically equivalent (each sample is its own
  document) and supports CPU + fp16 + autograd.
- Replace custom _RMSNorm with torch.nn.RMSNorm.
- Drop _ZUNAEncoderArgs dataclass, _build_zuna_encoder_args validation,
  InitStdFactor enum, and the six zero-arg config helpers in favour of
  module constants plus one patchable _encoder_config().
- Drop unused dataclass knobs: init_base_std, init_std_factor,
  encoder_hidden_dim, n_kv_heads, ffn_dim_multiplier, multiple_of,
  dropout_type / dropout_vec, encoder_latent_downsample_factor,
  tok_idx_type.
- Drop _apply_channel_mask + dropped_channels API (caller-side concern).
- Drop _discretize_channel_positions string extremes_type; inline the
  constant ZUNA_POS_HALF_RANGE = 0.12.
- Drop _repeat_kv / GQA plumbing (never used: n_kv_heads always equals
  n_heads in the published config).
- Cache positions from chs_info at construction; resolve once.
- Replace 4-lookup + cat RoPE gather with one indexed read + flatten.
- Bucket positions in fp32 so fp16 inference doesn't shift bucket edges.
- Distinguish 'no coords' vs 'names without montage' errors.
- Split forward shape validation so ndim / n_chans / n_times mismatches
  get targeted messages.

Param count matches Zyphra/ZUNA published checkpoint exactly
(172,069,668 params for n_chans=22, n_outputs=4).

Tests (test/unit_tests/models/test_models.py, 290 -> 151 lines):
- Drop _zuna_forward inference_mode wrapper - no longer needed.
- Drop @pytest.mark.skipif(not zuna.HAS_FLEX) on every test.
- Drop redundant channel_mask / dropped_channels tests with the API.
- Drop _zuna_published_config snapshot test (superseded by inline
  constants).
- Parametrize n_times rejection over (1279, 1281).
- Add test_zuna_requires_montage_when_names_only.
- Drop CUDA-only skips in test_integration.py for ZUNA.