feat(attention): add unfused compressed sparse attention

[FDecaYed]

What

Add the unfused SBHD compressed sparse attention core:

• compressor and learned indexer;
• sliding-window and compressed-KV selection;
• sparse-attention execution and dense-reference mode;
• inert CSA configuration fields;
• focused helper, forward/backward, compressor/indexer, dense-mode, and RoPE coverage.

This slice deliberately does not enable the dsv4_hybrid model variant. Its Files changed view is limited to the algorithm, configuration fields, and tests.

Stack

This PR is stacked on #5944 because CSA consumes its explicit MLA RoPE API and remove-interleaving behavior.

• feat(attention): add SBHD DSv4 hybrid orchestration #5948 enables SBHD-only DSv4 Hybrid attention orchestration.
• fix(pipeline): scale DSv4 hybrid indexer loss #5942 is an independent semantic merge prerequisite for final end-to-end indexer-loss scaling, but is not a code-base dependency of this PR.

Provenance

• Primarily reconstructs the CSA portion of [dev] [DeepSeek-v4] Part 1: Hybrid Attention with CSA and HCA #4458.
• Includes the relevant dense-loss and rotary corrections from [dev] [fix] [DeepSeek-v4] fix dense loss and rope type in DSv4 Hybrid Attention #5018.
• Refactors the frozen feature set collected in [Tracking] DeepSeek-v4 HybridModel merge plan #5795.
• Original implementation by the source PR authors; DSv4 integration by @hxbai.
• Reconstructed for current main; feature cutoff: 2026-07-21.
• This is not a mechanical replay or rebase.

Deliberately excluded

• model-variant selection and Hybrid attention orchestration;
• fused CSA/DSA kernels;
• packed-sequence/THD and context-parallel support;
• HybridModel and GPTModel wiring;
• HCA, Hash MoE, mHC, and FP8 marker integration.

Testing

• isort, ruff, Black, Python compilation, and git diff --check passed for the exact slice.
• Focused pytest collection is blocked locally because this host lacks Torch/CUDA; PR CI owns execution.

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[FDecaYed]

/ok to test bb891cb

[FDecaYed]

Closing this draft until Round 1. Its logical successor is D1 (unfused SBHD compressed sparse attention), rebuilt after P1 and P6 merge. It will target main directly rather than remain stacked. Tracking PR: #5795.