feat(qwen3-next): add pipeline-parallel stage support

[inureyes]

Summary

Adds pipeline-parallel (PP) stage support for qwen3_next, which was the only Qwen3-Next-style hybrid family missing it (qwen3_5 already has full PP support and reuses qwen3_next's building blocks).

Changes

• Qwen3NextStageModel in src/models/qwen3_next.rs, modeled on Qwen35StageModel: loads only the layers in filter.layer_range, accepts/produces hidden states on intermediate stages and token IDs / final logits on the first/last stage, and ties embeddings on the final stage.
• Qwen3NextStageExecutor in src/distributed/pipeline/stage_executor/qwen3_next.rs, modeled on qwen35.rs.
• Registration in src/distributed/pipeline/stage_executor/mod.rs: the Qwen3Next StageFamily variant, name mapping, PP-capable set, and ModelType dispatch.

Correctness

qwen3_next mixes linear (gated-delta, ArraysCache) and attention (KVCache) layers. The stage model builds and advances the correct cache type per local layer and derives the SSM/attention masks from the layers actually present in the stage, using global layer indices for typing. Covered by new unit tests: stage layer typing on global indices, per-stage attention offset (first-stage local attention and the all-linear default), and cache snapshot/restore round-trips for both the linear and attention caches.

Validation

• cargo check --lib --tests --features metal,accelerate: compiles clean.
• cargo test --release --features metal,accelerate qwen3_next: 10 tests pass (helper layout, stage typing/offset, cache round-trips, plan-generator replication, kv-arch interval, family registration).
• cargo test --release --features metal,accelerate family_registry: 5 pass.
• Not validated here: a real multi-stage run on an actual qwen3_next checkpoint. No qwen3_next checkpoint is present in this environment (models/ is unpopulated), so the end-to-end multi-stage decode parity against single-process decode was not executed and should be confirmed on hardware with a real checkpoint before relying on it in production.

Closes #510