feat(phi3-small): implement blocksparse attention

[inureyes]

Summary

Replaces the dense-attention fallback on Phi-3-small's non-dense layers with the trained blocksparse pattern (local blocks plus a vertical stride), while the periodic fully-dense layers keep dense attention. This restores long-context fidelity and the intended long-sequence efficiency; short context is unchanged.

Changes

• Blocksparse mask / gather for layers where the per-layer block_sparse flag is set: attend to the local window of blocksparse_num_local_blocks blocks of size blocksparse_block_size ending at the current query block, plus earlier blocks selected by blocksparse_vert_stride.
• Dense path preserved unchanged for the every-dense_attention_every_n_layers dense layers.
• The already-computed per-layer block_sparse flag is threaded into the attention path so dense and sparse layers diverge correctly.
• The L==1 decode path gathers the selected key blocks instead of allocating a full sparse mask each step.

Correctness (all covered by unit tests)

• short_context_mask_equals_causal and short_context_output_matches_dense_fallback: when the sequence fits within the local window the blocksparse path reduces to the dense fallback.
• blocksparse_mask_matches_reference_pattern and blocksparse_selected_key_blocks_are_local_plus_vertical: the selected key-block set matches the reference (local blocks + vertical stride) on a constructed case.
• long_context_mask_is_sparser_than_causal and long_context_output_diverges_from_dense: long context follows the trained sparsity, not dense.
• decode_single_row_mask_follows_pattern: the L==1 decode gather follows the pattern.
• block_sparse_flag_marks_every_nth_layer_dense, config_parses_blocksparse_fields, config_defaults_apply_when_blocksparse_fields_absent.

Validation

• cargo check --lib --tests --features metal,accelerate: compiles clean.
• cargo test --release --features metal,accelerate phi3small: 10 tests pass.
• Not validated here: a real Phi-3-small checkpoint generating coherently beyond the local-block window. No Phi-3-small checkpoint is present in this environment (models/ is unpopulated), so end-to-end long-context generation was not executed and should be confirmed on a real checkpoint before relying on it in production.

Closes #511