refactor(loader): gate batched-prefill MLA exclusion on capability, not model_type

[rsafier]

Stacked on #260 (base fix/generic-nvfp4-moe-gate). Draft until the e2e co-dispatch smoke lands (unit tests already green).

What

check_kernel_batched_eligible() blocked batched-prefill attention for MLA models via config.model_type == "mistral" — a hardcoded proxy the original author flagged as conservative ("mistral is the only MLA model in Atlas today"). This keys it on the actual capability: config.kv_lora_rank > 0 (== AttentionType::Mla).

Behavior preservation

kv_lora_rank > 0 == {mistral, deepseek_v4}; every other model_type has kv_lora_rank == 0 → is_mla=false → unchanged. deepseek_v4 is the only newly-matched model, but it is already rejected one check later by head_dim > 256 (head_dim=512) — so its outcome (not batched-eligible) is identical. Net: bit-identical on every shipped model, plus it future-proofs any MLA checkpoint shipped with head_dim <= 256. batch_kernel_tests pass (12/12) — they directly exercise the converted predicate (MLA reject, head_dim reject, non-MLA accept).

Why this is the only additional conversion

This PR is the output of an exhaustive adversarial pass over every model_type string gate in the tree (stacking on #260, which already did the two qwen35/load_layers.rs MoE gates). The analysis found this is the only other safe architecture-generic conversion. In particular it rejected several tempting ones:

• MTP allowlist (preflight.rs MTP_SUPPORTED_MODEL_TYPES) → must NOT become capabilities().has_mtp: has_mtp (mtp_num_hidden_layers>0) is broader than the allowlist and is true for deepseek_v4 / minimax_m2 / step3p7 — converting would re-enable exactly the unconsumable per-module-MTP loader path (MiniMax) the guard exists to reject.
• is_qwen35* predicates → loader sub-dispatch (protected; no capability expresses "use dense vs VL loader").
• gemma4 skip_lm_head_quantization / softcap, minimax/step3p7 setup, the dispatch.rs model_type→kernel-dir rewrites → genuinely checkpoint/family-specific, kept.

No kernels move; no loader dispatch changes. The refactor is deliberately minimal — the value is as much the documented boundary of what stays model_type-gated as the one conversion itself.

[rsafier]

Validation — PASS

• Unit: cargo test -p spark-model batch_kernel → 12/12 pass (directly exercises the converted predicate: MLA-reject with is_mla=true, head_dim-reject with is_mla=false, non-MLA accept).
• Build: release rc=0 (all targets), fmt + the change compiles every call site.
• e2e regression (Holo-3.1-35B-A3B-NVFP4, non-MLA / kv_lora_rank=0): served with ATLAS_PREFILL_CODISPATCH=1, 2 concurrent prompts → both coherent (correct Rayleigh-scattering answers), batched-prefill path engages (unchanged), 0 crashes. Confirms the MLA guard does not fire on non-MLA models and the batched path is intact.

deepseek_v4 (the one newly-is_mla model) remains not-batched-eligible exactly as before (head_dim=512 > 256 already rejected it); no MLA checkpoint with head_dim<=256 is shipped today, so no observable delta on any current model.

[SeedSource]

Approve (SeedSource, CODEOWNER) — batched-prefill MLA-exclusion keyed on is_mla (kv_lora_rank>0) instead of model_type=="mistral". More correct (covers deepseek_v4 explicitly; it was already caught one check later by head_dim>256, so behavior is unchanged on every shipped model) and off the string-compare on a per-prefill hot path. Tests updated consistently. CI green. Reviewed by code-read + CI; not behaviorally re-gated.