Engine Inference Core E3: qwen3_5_moe, lfm2, nemotron_h

[tlkahn]

E3 - qwen3_5_moe, lfm2, nemotron_h

Remaining non-dense generate families: MoE+linear-attn (qwen3_5_moe) and hybrid conv/SSM (lfm2, nemotron_h).

Spec: doc/plans/engine-inference-core.md § E3

Part of: #56
Depends on: E1 (#104) for qwen3_5_moe MoE block; hybrid cache is E3-local (#117)

Tracking notes folded into sub-issues

• Extend weights_expected_names() per new family (pattern: qwen3_layer_matmuls / qwen3_layer_norms in weights.c) - owned by each family issue
• Unify hybrid/dense gate checks in engine_model_check_supported when linear attention lands (PR feat(engine): stage D6 - qwen3_5 dense full-attn deltas #99 finding 6) - owned by E3: Hybrid cache (SSM/conv) + unified hybrid support gate #117

Sub-issues

• E3: Hybrid cache (SSM/conv) + unified hybrid support gate #117 Hybrid cache (SSM/conv state) + unified hybrid support gate (foundation)
• E3: qwen3_5_moe (linear attention + MoE FFN) #118 qwen3_5_moe (MODEL_QWEN3_5_MOE)
• E3: lfm2 (short-conv + attention hybrid) #119 lfm2 (short-conv + attention hybrid)
• E3: nemotron_h (mamba-style SSM + attention/MLP) #120 nemotron_h (mamba-style SSM interleaved with attention/MLP)
• E3: Parity gates (qwen3_5_moe, lfm2, nemotron_h) #121 Parity gates per family

Suggested order

1. E3: Hybrid cache (SSM/conv) + unified hybrid support gate #117 hybrid cache + gate unification
2. E3: qwen3_5_moe (linear attention + MoE FFN) #118 / E3: lfm2 (short-conv + attention hybrid) #119 / E3: nemotron_h (mamba-style SSM + attention/MLP) #120 family forwards (parallelizable after E3: Hybrid cache (SSM/conv) + unified hybrid support gate #117; E3: qwen3_5_moe (linear attention + MoE FFN) #118 also needs E1)
3. E3: Parity gates (qwen3_5_moe, lfm2, nemotron_h) #121 parity as each family lands

Acceptance

• Each family generates on a canonical small checkpoint (skip-if-absent)
• Hybrid cache correctly hosts SSM/conv entries alongside KV entries
• Pure-dense rejects no longer use the Stage D triple-gate once hybrid is first-class