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

[rsafier]

Re-opened replacement for #261. #261 was merged into its base feature branch by mistake on 2026-07-09 and reverted; the base branch has since been restored and restacked onto main. GitHub cannot reopen a merged PR, so this is a fresh PR carrying the identical commit (verified patch-identical via git range-diff). Marked ready for review — the original's "draft" wording was stale.

Stacked on #260 (base fix/generic-nvfp4-moe-gate). Unit tests green; the e2e co-dispatch smoke is still outstanding.

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.

[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: low - serve-test-required (triage advisory): False - path-heuristic (authority): True
• Rationale: Refactoring MLA eligibility check from string model type to boolean capability flag; no quantization or serving changes.
• Serving-path files: crates/spark-model/src/model/trait_impl/prefill_b/batch_kernel/eligible.rs, crates/spark-model/src/model/trait_impl/prefill_b/batch_kernel_tests.rs

Serve gate

• Build: Nones - Qwen3.6-35B-A3B-FP8 on GB10 node1
• Top-k KL vs main: None - decode delta None%
• Coherence probe: None (note: our test_coherence.py is flaky here; KL is the reliable serve signal)

GLM-5.2 code review

Correctness: Sound. kv_lora_rank > 0 is the canonical MLA marker and correctly supersedes the brittle model_type == "mistral" string match. The comment accurately notes deepseek_v4 was already caught by the subsequent head_dim > 256 guard, so no behavioral regression on shipped models. Parameter type change from &str to bool is clean and all call sites are updated.

Risk: Low. The refactor is narrowly scoped to one eligibility gate. The only latent risk: if a future MLA model has kv_lora_rank > 0 and head_dim <= 256, it would now be rejected earlier (correct) rather than slipping through the string check (bug). That's a strict improvement.

Quality concerns:

1. The comment block (lines 160-165) is verbose for a hot-path file. Trim to 2 lines.
2. No test exercises a non-mistral MLA model (the stated motivation). Add one with is_mla=true, head_dim=128 to lock in the fix.
3. Tests pass false for non-MLA cases but never assert the kv_lora_rank derivation logic itself—consider a unit test on the caller side.

Verdict: Approve with the test gap addressed.

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