fix(deepseek-v4): main θ=10000 rope on sliding_attention layers (was θ=160000)

[SeedSource]

fix(deepseek-v4): main θ=10000 rope on sliding_attention layers (was θ=160000)

Stacked on Avarok-Cybersecurity#275 (feat/v4-flash-compressed-kv-decode-inc3) — this touches the V4-Flash attention
rope, which lands via Avarok-Cybersecurity#275; it's meaningless without the port under it. Retarget to main once Avarok-Cybersecurity#275 merges.

The bug

DeepSeek-V4 uses two RoPE bases, per layer type:

• sliding_attention layers (compress_ratios[L]==0) → main rope, rope_theta=10000, no YaRN;
• compressed_sparse_attention / heavily_compressed_attention → compress rope, compress_rope_theta=160000 + YaRN.

Reference modular_deepseek_v4.py (config): "yarn is applied ONLY to layers with a compressor (CSA/HCA); …
main = pure." Attention selects rope_layer_type = "main" if layer_type=="sliding_attention" else "compress".

Atlas builds a single shared yarn_inv_freq (θ=160000 + YaRN, ensure_yarn_inv_freq) and applies it to the
Q/K rope and the eq.26 output de-rotation on every layer. So the 3 sliding layers (indices 0, 1, 42 in this
checkpoint) rotate with θ=160000 where they must use plain θ=10000.

The error is layer-type-dependent and position-growing (a base-θ difference, not YaRN interpolation), which is
why it was invisible to short-context probes:

decode position	max per-dim angle error (θ=160000 vs θ=10000)	|Δcos|
150	14.5 rad	—
240	23.2 rad	—
330	31.9 rad	up to 1.85 (of 2.0 max)

The fix

Add main_inv_freq (plain θ=10000, mscale=1) to MlaWeights; select it over yarn_inv_freq when
mla.compressor.is_none() (== sliding layer) at all four raw-arm rope sites — rope-in and the eq.26
de-rotation, in both prefill (cache_skip_v4.rs) and single-token decode (attention_forward_v4.rs). The
de-rotation must match the rope-in inv_freq or the output is scrambled. CSA/HCA layers are unchanged (still
θ=160000 + YaRN). Non-V4 (mistral) loader sets main_inv_freq = yarn_inv_freq (no split). 6 files, +77/−8.

Measured (fp8-KV EP=2, greedy, RoCE-verified; q3/q8 futures risk-sizing gates)

gate	before (θ=160000 all layers)	after (this fix)
q3 (max size)	"10 points / $10,000" — wrong	"1 contract … take the trade" — correct (matches NVIDIA NIM reference)
q8 (revenge)	fabricates a "$5,000 margin" not in the scenario	recalls the real $1,600 / $2,000; verdict still "valid" (should be not-valid)
coherence onset (280-tok non-arithmetic probe)	~150 tok	~150 tok (unchanged)

Scope of the claim: this fixes the sliding-layer rope fault — q3 flips to the correct answer, q8 stops
fabricating numbers. It is not a full task fix: q8's multi-step breach reasoning still errs, and the ~150-token
coherence ceiling is unmoved by this change (measured above) — a separate, still-open issue, not the rope.

Coverage gap (batched / MTP)

This PR splits the rope only on the single-token decode + prefill paths. The batched decode
(trait_impl/multi_seq/mla.rs) and MTP verify paths still apply yarn_inv_freq (θ=160000) to sliding layers.

Observable consequence: single-token (--max-batch-size 1) output on layers 0/1/42 is now correct; batched
decode and MTP-verify output will differ on those layers until they get the same compressor.is_none() split. A
follow-up will cover them; flagged here so the partial coverage is explicit, not silent.

cargo check + clippy (deny-all) + fmt clean.

🤖 Generated with Claude Code

[github-actions]

Thanks for the contribution. Before we can merge, please sign our Contributor License Agreement by replying to this comment with exactly:

I have read the CLA Document and I hereby sign the CLA

---

Thanks for the contribution. Before we can merge, please sign our Contributor License Agreement by replying to this comment with exactly:

I have read the CLA Document and I hereby sign the CLA

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[SeedSource]

Wrong target — this was against our own fork (SeedSource), invisible to Avarok maintainers. inc3 (base) exists only on the fork, so a cross-fork stacked PR to Avarok can't target it. Holding the rope fix on branch fix/v4-sliding-layer-rope-theta (pushed, verified: 6 files, +77/-8, fmt+clippy-deny-all+compile clean); will open against Avarok-Cybersecurity/atlas base=main once Avarok-Cybersecurity#275 lands inc3→main (one-command retarget was the plan anyway). Body/evidence/stack-note carry over.