fix(loader): gate CUTLASS grouped NVFP4 MoE on architecture, not model_type string

[rsafier]

Draft until the runtime regression check lands (building now — validating Holo-3.1-35B and AgentWorld both engage CUTLASS grouped + stay coherent on the actual binary).

Problem

The fast CUTLASS grouped NVFP4 MoE path (opt-in via ATLAS_HOLO_LOW_MEMORY_MOE=1) was gated on config.model_type == "holo3_1_moe", so any other NVFP4-MoE checkpoint silently fell back to the slow moe_w4a16 path even with the flag set. Concrete repro: sakamakismile/Qwen-AgentWorld-35B-A3B-NVFP4 (model_type qwen3_5_moe, compressed-tensors NVFP4) — no CUTLASS grouped SFB signal, ran moe_w4a16.

But holo3_1_moe / qwen3_5_moe / qwen3_6_moe are the same architecture at the weight level — factory::loader_for_config maps all three to Qwen35WeightLoader, and holo3_1_moe is a label fabricated at config/dispatch.rs:120-128 (rewrites qwen3_5_moe→holo3_1_moe when image_token_id==248056) purely so kernel-target directory lookup can disambiguate 3.5 vs 3.6 (they collide on hidden=2048). Downstream code then mistook that label for an architecture discriminator.

Fix (2 gates, one file — qwen35/load_layers.rs)

• holo_nvfp4_moe → nvfp4_moe = config.num_experts > 0 && quant_format == Nvfp4 (architecture, not label).
• is_holo_modelopt_mixed_precision → is_modelopt_mixed_precision: drop the model_type == "holo3_1_moe" conjunct; keep the quant-config test (any Qwen3.5/3.6-family MoE shipping modelopt MIXED_PRECISION is now recognized).

Scope / safety

• No kernels move. The config/dispatch.rs model_type rewrites (needed for kernel-directory resolution) are deliberately untouched — this only changes which MoE dispatch fires, not kernel layout.
• No regression on Holo: holo3_1_moe still qualifies (num_experts=256, Nvfp4), and the mixed-precision path still fires for Holo (modelopt MIXED_PRECISION). It now also fires for architecturally-identical siblings shipping the same quant format — no existing checkpoint changes behavior (we ship no non-Holo modelopt-MIXED MoE).
• This is the focused bug fix. A broader capability-based refactor (replacing model_type string checks across the codebase — MTP allowlist, MLA guard, etc.) is a separate, larger effort scoped for a follow-up.

[rsafier]

Runtime regression — PASS (validated on the actual binary, both models)

Built spark-genmoe (main + this fix, ATLAS_TARGET_MODEL=*), served each single-node with ATLAS_HOLO_LOW_MEMORY_MOE=1:

AgentWorld (qwen3_5_moe, compressed-tensors NVFP4) — the fix:

• CUTLASS grouped SFB: built 256 experts ×40 layers (was falling back to moe_w4a16 before)
• Coherent: "The capital of France is Paris."

Holo-3.1-35B-A3B-NVFP4 (holo3_1_moe, modelopt MIXED_PRECISION) — no regression:

• CUTLASS grouped SFB ×40 layers (unchanged)
• is_modelopt_mixed_precision still fires: "routing Holo ModelOpt SSM/attention projections through native FP8", "Layer N: SSM native FP8 — w8a16" — the modelopt path is byte-for-byte the same
• Coherent: "15 plus 27 equals 42."

No kernels moved; config/dispatch.rs rewrites untouched. cargo check + fmt clean.

[SeedSource]

[BETA] Automated PR review — SeedSource AI pipeline

Advisory only, not a merge gate. AI-generated (internal risk-triage, Atlas serve gate, GLM-5.2 review), attributed per Atlas convention.

Triage

• Risk: med - serve-test-required (triage advisory): True - path-heuristic (authority): True
• Rationale: Broadens NVFP4 MoE and modelopt mixed-precision detection, removing model_type gates to support more checkpoints.
• Serving-path files: crates/spark-model/src/weight_loader/qwen35/load_layers.rs

Serve gate

• Build: 18s - Qwen3.6-35B-A3B-FP8 on GB10 node1
• Top-k KL vs main: 0.0001 (no shift vs main) - decode delta -0.0%
• Coherence probe: coherence=pass ping=10/10 (note: our test_coherence.py is flaky here; KL is the reliable serve signal)

GLM-5.2 code review

Correctness: Sound. Gating on num_experts > 0 + QuantFormat::Nvfp4 is the right architectural check; removing the model_type == "holo3_1_moe" string match correctly unblocks sibling/third-party checkpoints routed through Qwen35WeightLoader. The is_modelopt_mixed_precision rename and de-gating is consistent — the quant_config fields are sufficient and the model_type check was redundant exclusion.

Risk: Moderate. The broadened nvfp4_moe predicate now activates the CUTLASS grouped NVFP4 MoE transpose path for any NVFP4 MoE checkpoint. If a non-Holo NVFP4 MoE has expert weight layouts that differ from what the grouped path expects (e.g., different packing, scaling block size), this will silently route through an untested path rather than the known-safe moe_w4a16 fallback. The env-var gate (ATLAS_HOLO_LOW_MEMORY_MOE=1) provides a mitigation, but only if operators know to set it.

Quality: Comment updates are accurate and well-scoped. eq_ignore_ascii_case("MIXED_PRECISION") is pre-existing and technically wrong (ASCII-case-insensitive compare against an uppercase literal matches lowercase but not mixed-case like "Mixed_Precision") — not introduced here, but worth a follow-up.

_{Pipeline: build+serve on GB10, GLM-5.2 via NVIDIA NIM, internal risk-triage classifier. External diff treated as untrusted input. Public repo.}